Nowadays, the structure and properties of water supply system are becoming more and more complex, and modeling is becoming increasingly difficult. In this paper, a data-driven subspace identification method is presented. Firstly, a brief introduction for water supply systems is presented; after that, we get the I/O data through the InfoWorks WS software; at last, a data-driven subspace identification method is applied to get the model. The result of the experiment shows the effectiveness of the proposed method.