diff --git a/news/370.bugfix.md b/news/370.bugfix.md
new file mode 100644
index 0000000000000000000000000000000000000000..43bf865658248dcd22952fcadf8b2d8d74b5ce7a
--- /dev/null
+++ b/news/370.bugfix.md
@@ -0,0 +1 @@
+Don't overwrite existing dependencies when importing from requirements.txt.
diff --git a/pdm/cli/actions.py b/pdm/cli/actions.py
index c1da2d3f5d4d74e68745e51823e267fd47c8e538..14f8edca9851caa359d802c53c31122991baf053 100644
--- a/pdm/cli/actions.py
+++ b/pdm/cli/actions.py
@@ -17,6 +17,7 @@ from pdm.cli.utils import (
     find_importable_files,
     format_lockfile,
     format_resolution_impossible,
+    merge_dictionary,
     save_version_specifiers,
     set_env_in_reg,
     translate_sections,
@@ -537,8 +538,8 @@ def do_import(
             tomlkit.comment("See https://www.python.org/dev/peps/pep-0621/")
         )
 
-    pyproject["project"].update(project_data)
-    pyproject["tool"]["pdm"].update(settings)
+    merge_dictionary(pyproject["project"], project_data)
+    merge_dictionary(pyproject["tool"]["pdm"], settings)
     pyproject["build-system"] = {
         "requires": ["pdm-pep517"],
         "build-backend": "pdm.pep517.api",
diff --git a/pdm/cli/options.py b/pdm/cli/options.py
index bb3e1c869efd32ba23a6f1f88ac5cd94810e76b1..dddc9d91d00b9a942975b78f3e8bc5c5b26e1477 100644
--- a/pdm/cli/options.py
+++ b/pdm/cli/options.py
@@ -102,7 +102,7 @@ sections_group.add_argument(
     dest="default",
     action="store_false",
     default=True,
-    help="Don't include dependencies from default seciton",
+    help="Don't include dependencies from default section",
 )
 
 
diff --git a/pdm/cli/utils.py b/pdm/cli/utils.py
index 387cba7c543e982bd275de0928511a032d3b7b0d..5a97b0f1b25b06f980d9aa906bdb10893d398792 100644
--- a/pdm/cli/utils.py
+++ b/pdm/cli/utils.py
@@ -462,3 +462,16 @@ def translate_sections(
     if default:
         sections.add("default")
     return sections
+
+
+def merge_dictionary(target: dict, input: dict) -> None:
+    """Merge the input dict with the target while preserving the existing values
+    properly. This will update the target dictionary in place.
+    """
+    for key, value in input.items():
+        if key not in target:
+            target[key] = value
+        elif isinstance(value, dict):
+            target[key].update(value)
+        elif isinstance(value, list):
+            target[key].extend(value)
diff --git a/pdm/formats/requirements.py b/pdm/formats/requirements.py
index 0b73312734ca587071c7943a37b48f32bf9edfa0..189c6e2e1704ce6f209f174e78eea7f3d9aeed95 100644
--- a/pdm/formats/requirements.py
+++ b/pdm/formats/requirements.py
@@ -7,7 +7,6 @@ from typing import Any, Dict, List, Optional, Tuple, Union
 from distlib.wheel import Wheel
 from pip._vendor.packaging.requirements import Requirement as PRequirement
 
-from pdm.exceptions import PdmUsageError
 from pdm.formats.base import make_array
 from pdm.models.candidates import Candidate
 from pdm.models.environment import Environment
@@ -109,12 +108,10 @@ def convert(
         reqs = [ireq_as_line(ireq, project.environment) for ireq in ireqs]
 
     deps = make_array(reqs, True)
-    data = {"dependencies": []}
+    data = {}
     settings = {}
-    if options.dev and options.section:
-        raise PdmUsageError("Can't specify --dev and --section at the same time")
-    elif options.dev:
-        settings["dev-dependencies"] = {"dev": deps}
+    if options.dev:
+        settings["dev-dependencies"] = {options.section or "dev": deps}
     elif options.section:
         data["optional-dependencies"] = {options.section: deps}
     else:
diff --git a/tests/cli/test_cli.py b/tests/cli/test_cli.py
index ee132006dbedf10fcd86e7702355864fc4009187..1ac15b5e46087da73a6ba5d95031f42a539e4680 100644
--- a/tests/cli/test_cli.py
+++ b/tests/cli/test_cli.py
@@ -291,6 +291,17 @@ def test_import_other_format_file(project, invoke, filename):
     assert result.exit_code == 0
 
 
+def test_import_requirement_no_overwrite(project, invoke, tmp_path):
+    project.add_dependencies({"requests": parse_requirement("requests")})
+    tmp_path.joinpath("reqs.txt").write_text("flask\nflask-login\n")
+    result = invoke(
+        ["import", "-dsweb", str(tmp_path.joinpath("reqs.txt"))], obj=project
+    )
+    assert result.exit_code == 0, result.stderr
+    assert list(project.get_dependencies()) == ["requests"]
+    assert list(project.get_dependencies("web")) == ["flask", "flask-login"]
+
+
 @pytest.mark.pypi
 def test_search_package(project, invoke):
     result = invoke(["search", "requests"], obj=project)
diff --git a/tests/test_formats.py b/tests/test_formats.py
index e39fcf32b0308a37cc8e6cf36fe7b821ab8e16d8..02bea85fa614b06777fa775f4b105e90a913427e 100644
--- a/tests/test_formats.py
+++ b/tests/test_formats.py
@@ -151,5 +151,5 @@ def test_import_requirements_with_section(project):
     assert "webassets==2.0" in section
     assert 'whoosh==2.7.4; sys_platform == "win32"' in section
     assert "-e git+https://github.com/pypa/pip.git@master#egg=pip" in section
-    assert not result["dependencies"]
+    assert not result.get("dependencies")
     assert not result.get("dev-dependencies", {}).get("dev")
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 72d7bad97472d9d6e6e00fa3ec0ab2a613eefaf0..357ca1dec5ba3d3436e7f4a95839dc07f22fcae5 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -5,6 +5,7 @@ import sys
 import pytest
 
 from pdm import utils
+from pdm.cli import utils as cli_utils
 
 
 @pytest.mark.parametrize(
@@ -66,3 +67,21 @@ def test_find_python_in_path(tmp_path):
     )
 
     assert not utils.find_python_in_path(tmp_path)
+
+
+def test_merge_dictionary():
+    target = {
+        "existing_dict": {"foo": "bar", "hello": "world"},
+        "existing_list": ["hello"],
+    }
+    input_dict = {
+        "existing_dict": {"foo": "baz"},
+        "existing_list": ["world"],
+        "new_dict": {"name": "Sam"},
+    }
+    cli_utils.merge_dictionary(target, input_dict)
+    assert target == {
+        "existing_dict": {"foo": "baz", "hello": "world"},
+        "existing_list": ["hello", "world"],
+        "new_dict": {"name": "Sam"},
+    }