An experiment table of variable flow air conditioning system was set up for studying the operation characteristics of variable primary flow air-conditioning system under partial load conditions with constant differential pressure control strategy. The results show that as the load rate decreases, the COP of the chiller increases first and then decreases, and the highest COP appears at 60% load rate. The integrated efficiency of the pump is always reduced, while the chiller input power and pump input power show different trends-the pump input power is always reduced, and the chiller input power decreases first and then increases, resulting in the total input power of chiller and pump at 27.2% load rate being higher than that at 45.1% load rate. The variable frequency operation of the pump does not satisfy the affinity law. The equation of correlations between pump input power and flow rate is obtained, reasoning out that pump input power is related to the ratio of the constant differential pressure, the resistance of the equipment room and the integrated efficiency of the pump.The input power of the pump in this experiment is approximately proportional to the second power of the flow rate.