Flexible multi-state switch (FMSS) has the characteristics of nonlinearity and strong coupling. It is difficult to achieve complete decoupling of output power through traditional proportional integral (PI)control. There are some problems in PI control such as difficulty in adjusting PI parameters,slow system dynamic response speed,and poor robustness. Firstly,feedback accurate linearization is uesd to transform the FMSS from a high-order nonlinear system into two completely independent first-order linear systems,realizing the complete decoupling of active and reactive currents. Then,aiming at the problems of slow approach speed,poor system convergence performance and large motion chattering of traditional sliding mode control,an improved sliding mode control based on adaptive approach rate is designed to optimize the voltage outer loop and the current inner loop,and the stability of the proposed algorithm is verified by Lyapunov function. Finally,the simulation analysis of the three-port FMSS are carried out in the Simulink software under the conditions of output power disturbance,AC voltage sag and system parameter perturbation. The results show that the proposed control enhances the robustness and anti-interference ability of the system,and it can effectively increase the dynamic response speed of the system.