The hydraulic characteristics and energy-saving performance of a chilled water system with primary pumps is analyzed. It is proved that the hydraulic characteristics of the system with terminal of the air conditioners tends to large temperature differences and small flow rate, while the hydraulic characteristics of the system with terminal of the combination of fan coil units and air conditioners tends to large flow rate and small temperature differences. When the cooling load is reduced, in order to ensure the energy-saving performance of the primary pump system, the fixed chilled water flow is required. If the fixed water flow system is adopted to regulate the outlet water temperature，the bypass water flow rate should be decreased as much as possible. If the pump frequency regulation is selected, and the proportion of energy consumption of the pump accounts for 30% of the total at the same time, the more the system’s hydraulic characteristics approaches that of air conditioners, the higher the energy-saving rate will be, which could be markedly greater than 10%. By regulating the water temperature and cold water quantity, over flow of chilled water still exists. Therefore, bypass water flow should be provided. And the bypass pipe diameter is determined respectively by the water flows which are 1.2 ~ 1.8 or 0.2 ~ 1.0 times larger than that of a single chiller when the pump works in frequency conversion and constant frequency.