The present invention relates to a method for testing UPS performance, and especially to a method for dynamically testing UPS. The dynamic testing method of UPS comprises the steps of: abruptly load adding or abruptly load reducing testing step and characteristic switching step, in the step, the phase voltage and frequency in empty load and mesomeric state require to be measured in the abruptly loading adding or abruptly load reducing testing step; abruptly adding the load from 0% to 100% or abruptly reducing the load from 100% to 0% and measuring the output transient voltage of UPS; and determining whether the UPS is qualified. The characteristic switching step uses a storage oscilloscope and a pressure regulator which can simulates the variation of alternating current for testing whether a breaking point exists in power supplying when the alternating current is switched to the inverter in 100% of load. The adopted technical settling plan totally settles the problems of more complication and hard operation in the prior UPS dynamic measuring method thereby increasing the testing efficiency remarkably.