By analyzing the present status of water supply system of our country, the concept of subarea of city water supply system is detailed. Pointing out that the subarea is the foundation of optimal control. Then the method of subarea based on the boundary of water supply and the pressure of the nodes is brought forward, and how to programming between and in the small area is discussed. Through subarea, the functions of pipes are definite. So we can more easily compute the flow of some area, control the pressure of pipes, optimize the water supply system. Thus, the aim to reducing the pipe leaks and improving the benefit on economy is obtained.