The invention discloses a loop testing system used for carrying out thermal transient impact tests and high-temperature durability tests on a tested pump. The loop testing system comprises a cold source supply loop assembly, a hot source supply loop assembly and a controller, wherein the cold source supply loop assembly and the hot source supply loop assembly are connected to the tested pump through pipelines respectively; the controller is connected to the tested pump, an electric appliance part of the cold source supply loop assembly and an electric appliance part of the hot source supply loop assembly through circuits. The invention further discloses a testing method of the loop testing system. The system has the self-feedback adjusting function, test temperature and time requirements can be precisely controlled, and meanwhile operation safety is improved; a method for balancing the temperature through cold and heat mixing is adopted, the thermal transient impact tests and the high-temperature durability tests are realized in the same loop testing system, and the operation method is simple and convenient.