diff --git a/paddle/gserver/gradientmachines/GradientMachine.h b/paddle/gserver/gradientmachines/GradientMachine.h
index a814e771d12e3ae71212fd62c06a4f56338286ae..5469c0d89f49648743f3a7e851694e4e1e736fb2 100644
--- a/paddle/gserver/gradientmachines/GradientMachine.h
+++ b/paddle/gserver/gradientmachines/GradientMachine.h
@@ -134,7 +134,7 @@ public:
     backward(callback);
   }
 
-  virtual MatrixPtr getLayerOutput(const std::string& layerName) {
+  virtual MatrixPtr getLayerOutput(const std::string& layerName) const {
     return nullptr;
   }
 
diff --git a/paddle/gserver/gradientmachines/MultiGradientMachine.cpp b/paddle/gserver/gradientmachines/MultiGradientMachine.cpp
index 7e60920376d195c708ee5aafff25924aaabe17be..2d42e648302f1673ae067ded43db4502679a6625 100644
--- a/paddle/gserver/gradientmachines/MultiGradientMachine.cpp
+++ b/paddle/gserver/gradientmachines/MultiGradientMachine.cpp
@@ -282,7 +282,8 @@ void MultiGradientMachine::forwardBackward(const std::vector<Argument>& inArgs,
   backwardImp(callback);
 }
 
-MatrixPtr MultiGradientMachine::getLayerOutput(const std::string& layerName) {
+MatrixPtr MultiGradientMachine::getLayerOutput(
+    const std::string& layerName) const {
   // each thread has the same neural network
   auto nn = threads_[0]->getGradientMachine();
   size_t height = 0;
@@ -301,11 +302,10 @@ MatrixPtr MultiGradientMachine::getLayerOutput(const std::string& layerName) {
 
   // copy one layer output from one trainer thread at each time
   size_t startRow = 0;
-
-  for (size_t i = 0; i < threads_.size(); i++) {
-    auto tmpMatrix = layerOutput->subMatrix(startRow, mats[i]->getHeight());
-    tmpMatrix->copyFrom(*mats[i]);
-    startRow += mats[i]->getHeight();
+  for (auto& mat : mats) {
+    auto tmpMatrix = layerOutput->subMatrix(startRow, mat->getHeight());
+    tmpMatrix->copyFrom(*mat);
+    startRow += mat->getHeight();
   }
 
   return layerOutput;
diff --git a/paddle/gserver/gradientmachines/MultiGradientMachine.h b/paddle/gserver/gradientmachines/MultiGradientMachine.h
index 988d5098179806fc75aa2fae5dcc4330d7963257..a1a2d417062de5a1adeb83506b2a30339a816a13 100644
--- a/paddle/gserver/gradientmachines/MultiGradientMachine.h
+++ b/paddle/gserver/gradientmachines/MultiGradientMachine.h
@@ -189,7 +189,7 @@ public:
                        PassType passType,
                        const UpdateCallback& callback);
 
-  virtual MatrixPtr getLayerOutput(const std::string& layerName);
+  virtual MatrixPtr getLayerOutput(const std::string& layerName) const;
 
   virtual void onPassEnd();
 
diff --git a/paddle/gserver/gradientmachines/NeuralNetwork.cpp b/paddle/gserver/gradientmachines/NeuralNetwork.cpp
index 1f9ace4f67fdcab6af522277fa83bb0e6044360d..00887c81d47687f3f3f976c2050289a3c73885e9 100644
--- a/paddle/gserver/gradientmachines/NeuralNetwork.cpp
+++ b/paddle/gserver/gradientmachines/NeuralNetwork.cpp
@@ -293,7 +293,7 @@ void NeuralNetwork::backward(const UpdateCallback& callback) {
   }
 }
 
-MatrixPtr NeuralNetwork::getLayerOutput(const std::string& layerName) {
+MatrixPtr NeuralNetwork::getLayerOutput(const std::string& layerName) const {
   auto it = layerMap_.find(layerName);
   CHECK(it != layerMap_.end()) << "Cannot find layer: " << layerName;
   return it->second->getOutputValue();
diff --git a/paddle/gserver/gradientmachines/NeuralNetwork.h b/paddle/gserver/gradientmachines/NeuralNetwork.h
index bf9ed09327f2f13585a2b37993f3139fe6cb862b..6ecc251a409cb9daefce704dd89027b9bd03744c 100644
--- a/paddle/gserver/gradientmachines/NeuralNetwork.h
+++ b/paddle/gserver/gradientmachines/NeuralNetwork.h
@@ -87,7 +87,7 @@ public:
 
   virtual void backward(const UpdateCallback& callback = nullptr);
 
-  virtual MatrixPtr getLayerOutput(const std::string& layerName);
+  virtual MatrixPtr getLayerOutput(const std::string& layerName) const;
 
   const LayerPtr& getLayer(const std::string& layerName) const {
     auto it = layerMap_.find(layerName);