Abstract: Two newly purchased UPS power supplies of our plant's secondary control device have been running for a short time. It was found that UPS bypass voltage display fl uctuated on site. Combined with the previous UPS fault handling experience, this paper analyzes the output voltage fluctuation of UPS bypass voltage regulator cabinet from the aspects of equipment appearance inspection, electrical and mechanical inspection and field test, It is pointed out that the power grid harmonics are the main cause of UPS bypass output voltage fluctuation, and the feasible improvement countermeasures are put forward.