Aiming at the problem of bus voltage stability caused by uncertain factors such as load fluctuation and parameter perturbation in DC microgrid system, a super-twisting second-order sliding mode control strategy is proposed in this paper. Firstly, taking the three-phase AC-DC converter of DC microgrid as the main body, the outer voltage loop controller of three-phase AC-DC converter in DC microgrid is designed by using super twisting idea by reasonably selecting sliding mode variables. The controller not only retains the strong robustness of traditional sliding mode, but also effectively reduces the system chattering. Then, according to Lyapunov"s second law, the stability conditions of the second order sliding mode are given. Finally, the feasibility and correctness of the control strategy are verified by numerical simulation.