A performance testing platform for heat pump water heater of CO₂ trans-critical cycle was designed and then constructed.Its evaporator and gas cooler were both sleeve type heat exchanger.The optimum charging amount was determined through experimental study.With the optimum charging amount, the effects of throttle opening degree and gas cooler water flow on the performance of the system were studied experimentally.The results show that the change of charging amount has more influence on discharge pressure and COP (coefficient of performance) of the system, and its influence on the evaporation pressure is rather small.If the throttle opening degree is too small, the discharge pressure and energy consumption of the system will be increased rapidly.Only when the water flow of gas cooler is appropriate, the system can provide hot water to meet the requirements with higher COP.