Abstract: When the power system operates under non-ideal conditions, significant voltage fluctuations occur. The utilizatuion of a passivity-based control strategy in the shunt active power filter (SAPF) may not effectively and accurately regulate power quality. Besides, conventional sliding mode control is also prone to buffeting. In the light of this, a passive super-twisting second-order sliding mode control strategy is proposed in this paper, which is a combination of passivity-based control and super-twisting second-order sliding mode control with stronger resisting disturbance capacity. First of all, the Euler-Lagrange model is established based on positive and negative sequence separation, in accordance with the mathematical model of active power filter. Secondly, the passivity of model is analyzed, and a passivity-based controller is designed according to its passiveness. Meanwhile, the passivity-based controller is further optimized by using the super-twisting second-order sliding mode control, thereby enhancing the robustness and capacity of resisting disturbance. Finally, simulation experiments are carried out under both ideal condition and four non-ideal conditions, namely load mutation, load imbalance, grid voltage imbalance and single-phase voltage mutation. The simulation results verify the effectiveness and superiority of the passive super-twisting second-order sliding mode control strategy.