A fixed-time output voltage regulation algorithm based on variable gain second-order sliding mode control is proposed for DC-DC buck converters. Firstly, taking into account external interference and unknown modeling dynamics, a state space mathematical model of buck circuit with uncertainty is obtained using the average state space method. Secondly, a sliding mode variable with a relative order of 2 is constructed based on the obtained mathematical model, and a novel sliding mode control algorithm with variable gain is proposed to achieve a fixed-time output voltage modulation control with convergence time independent of the initial system value. In addition, it is concluded through Lyapunov analysis that the closed-loop system can achieve global fixed-time stability. Finally, the effectiveness of the proposed algorithm is verified by comparing it with existing algorithms through simulation.