As more and more domain specific big data become available, there comes a strong need on the fast development and deployment of deep learning (DL) systems with high quality for domain specific applications, including many safety-critical scenarios. In traditional software engineering, software visualization plays an important role to enhance developers' performance with many tools available. However, there are limited visualization supports existing for DL systems, especially in integrated development environments (IDEs) that allow a developer to visualize the source code of a deep neural network (DNN) and its graph architecture. In this paper, we propose DeepGraph, a visualization tool for visualizing and understanding a deep neural network. DeepGraph analyzes the training program to construct the graph representation of a DNN, and establishes and maintains the linkage (mapping) between the source code of the training program and its corresponding neural network architecture. We implemented DeepGraph as a PyCharm plugin and performed preliminary empirical study to demonstrate its usefulness for understanding deep nueral networks.