Abstract: With the country's vigorous development of urban rail transit, uninterruptible power supply(UPS)system has important research significance and value. In order to meet the requirements of the power quality regulations, the integration of the power factor correction(PFC) input stage is an important requirement for UPS systems that guarantee the power supply. When the traditional carrier modulation is used, the input current of the three-level Boost PFC is always zero within a certain input voltage range, resulting in poor power factor. Therefore, this paper proposes a variable duty cycle control method based on k-value modulation, which can not only solve the problems of traditional carrier modulation at high input voltages, but also further suppress the distortion of input current. The k-value modulation method is first introduced, and the principle of k-value on the power factor of the converter and the effective value of the inductor current are analyzed. On this basis, the k value is optimized by combining the voltage balancing strategy to solve the problem of unbalanced voltage on the DC side. Finally, in order to solve the problem that the input current is seriously distorted at high input voltages, a variable duty cycle control method based on k-value modulation is proposed, and the influences of variable duty cycle control on the power factor, output power, voltage ripple and inductor current peak of the converter are analyzed. Finally, the correctness of the proposed methods is verified by simulation and the construction of a low-power experimental platform.