The present invention discloses a control method for the system gain of a UPS, which includes the steps of: S1: detecting a PFC unit so as to determine whether to start an LLC, and if so, then going to step S2; otherwise, going to step S8; S2: starting the LLC; S3: detecting the mains supply so as to determine whether to switch from the mains supply to a battery, and if so, then going to step S4; otherwise, going to step S5; S4: performing a frequency-fixed adjustment on the LLC with an operating frequency larger than a resonating frequency by a preset magnitude while clearing an integral quantity within a first preset time T1; and performing a frequency-fixed adjustment on the LLC with an operating frequency equal to the resonating frequency while clearing the integral quantity between the first preset time T1 and a second preset time T2, and going to step S8; S5: determining whether the LLC-in-series resonant converter has an overload, and if so, then going to step S6; otherwise, going to step S7; S6: performing a frequency-fixed adjustment on the LLC-in-series resonant converter with the operating frequency equal to the resonating frequency while clearing the integral quantity; and going to the step S8; S7: performing a normal PI adjustment on the LLC-in-series resonant converter; and S8: Ending. The operation condition of the PFC is detected in real time, the LLC is started when the PFC becomes abnormal, and the LLC is adjusted in response to a load during operating so that the system gain is maintained in a monotonously step-down region of a ZVS region.