DC side voltage regulation control is one of the key technologies for the stable operation of shunt active power filters (SAPF). A DC side voltage composite control strategy based on improved fuzzy PI control combined with Fast Delay Signal Cancellation (FDSC) filtering is proposed to address the shortcomings of large system overshoot and poor SAPF compensation effect under unbalanced load conditions when using traditional Proportional Integral (PI) control for SAPF DC side voltage stabilization. Firstly, the topology and control principle of SAPF were introduced, and the reasons for asymmetric current distortion in the power grid under unbalanced load conditions were analyzed; Secondly, a fuzzy PI controller was established to combine fuzzy PI control with traditional PI control for segmented control, balancing the dynamic response and steady-state accuracy of the system. To improve the compensation effect of SAPF under unbalanced load conditions, it is proposed to use FDSC to filter out the sixth and second harmonics of DC side voltage under three-phase load unbalanced conditions; Finally, theoretical analysis and simulation results verify the progressiveness of the proposed SAPF DC side composite voltage stabilizing control.