The water flow of closed-loop heat exchanger coil influences the overall performance of water source heat pump systems(WSHP). Energy consumption model of water source heat pump systems and calculation method of optimal water flow are both established. Through the theoretical calculations of water source heat pump systems performance, the theoretical relationship between overall coefficient of performance(COP）and percentage of cooling water flow and temperature of the water source heat pump systems is gotten. In the given temperature conditions, the value of the system COP is optimal under a percentage of cooling water flow. A closed-loop water source heat pump test bench is established in Yangtze River. Then, the variable flow conditions experiments are both done in summer and winter, experimental results and theoretical results are consistent. The calculation method can provide a reference for further research of the performance of closed-loop water source heat pump systems.