The invention relates to a device and method for testing performances of a commercial circulating heat pump water heater. The device includes a water-inlet temperature test component, a water-outlet temperature test component, a three-way valve, a flowmeter and a water pump, wherein the water-inlet temperature test component and the water-outlet temperature test component are arranged at a water inlet of a tested machine and a water outlet of the tested machine respectively and are used for testing the water-inlet and water-outlet temperatures of the tested machine, a first inlet of the three-way valve is connected with a water-outlet pipeline of the tested machine, a second inlet of the three-way valve is connected with a constant-temperature-water water-inlet pipeline, the outlet of the three-way valve is connected with a standard water tank through a water-inlet pipeline of the standard water tank, the flowmeter is arranged on a water-inlet pipeline between the three-way valve and the standard water tank, and the water pump is arranged on a water-outlet pipeline of the standard water bank and is used for transporting heated medium water to the water outlet of the tested machine. The method for testing the performances of the commercial circulating heat pump water heater by using the device includes the steps of preparation, test operation and test calculation. According to the method and device, interference factors are reduced, the heat-pump heating capacity of the commercial circulating heat pump water heater can be accurately tested.