diff --git a/doc/design/scope.md b/doc/design/scope.md
index 8d5744c227b70bffd7025badb4f7e9e1d5a401c4..f2869bc254acba6e4571ad723aefbfc51562f56d 100644
--- a/doc/design/scope.md
+++ b/doc/design/scope.md
@@ -2,22 +2,42 @@
 
 ## Overview
 
-预期使用场景。
+Scope is an important concept in programming languages, which defines a program region that a set of bindings between names and entities applies. In a specific scope, a valid name is uniquely associated with an entity, such as a variable. And in another scope, this name may refer to other entity or nothing at all. It clearly restricts the visibility and validity of names in a program. Hence **Scope** is introduced to PaddlePaddle to manage variables in context. But different from the original abstract concept, Scope now becomes an object with two important attributes:
+
+- Scope is an association of a name to variable.
+- Variables in a parent scope can be retrieved from local scope.
+
+A detailed explanation of these two attributes goes as following.
 
-引出Scope的两个属性。
-    1. Scope是Variable的Container
-    2. Scope可以共享
 
 ## Scope is a Container of Variables.
 
-    * Scope contains Variables as it's data member.
-    * Scope contains methods that are used to manage Variables, such as Create/Get/Delete.
-    * every variable only belong to one certain Scope.
-    * Scope should destruct all Variables within it when itself is destructed.
-    * Variable can only be created by Scope.
-    * Variable can only be got from Scope.
+Scope is used to provide a running environment for Net.
+
+1. Scope mainly has Variables as it's data member.
+
+    Scope is a running environment for Net. Net should get all it need to do computation from a scope, such as data buffer, state(momentum) etc.
+    All these data/state can be abstracted and create as variable in Paddle, so the only thing Scope need to care about is Variable.
+1. Variable can only be created by Scope.
+1. Variable can only be got from Scope.
+1. Scope contains methods that are used to manage Variables, such as Create/Get/Delete.
+
+    Because we only need to care about Variable, we only need method to manage the lifecycle of Variable.
+    - `Create` is used to create a Variable by its name and add the mapping relation.
+    - `Get` is used to find a Variable by name.
+    - `Delete` is used to remove a Variable because sometimes we want to release memory or other resources.
+
+1. Every variable only belongs to one certain Scope.
+
+    Variable can not be shared between scopes, if we want to use variables from different scope we can use `Parent scope`.
+
+1. Scope should destruct all Variables within it when itself is destructed.
+
+    Because Variable can only be got from Scope, when destroying Scope, we also need to destroy all the Vars in it.
+
+1. Scope do not contain Operators and have no information to run them.
 
-    * Scope do not contains Operators and have no information to run them.
+    Net is designed to drive the computation, Scope is only used to provide a running environment.
 
 ```cpp
 class Scope {