Abstract: To realize the smooth switching of multi-port flexible multi-state switches (FMSS) when the feeder fails, this paper proposes a smooth switching strategy based on the linear active disturbance rejection control (LADRC). The LADRC is adopted to replace the PI controller of the DC voltage outer-loop, and the decoupled extended state observer is designed to adjust the controller parameters. The inner-loop given current value feedback is introduced to cut down the variation when the control mode converts, realizing the smooth switching of the FMSS control mode. The MATLAB/Simulink simulation results verify that the proposed method is able to significantly reduce the DC voltage variation, realize a smooth active power transition, and quickly restore stability when the faults occur at the joints of PQ or U_dcQ control ports in the process of the FMSS control mode switching. Briefly, the smooth switching of multi-port FMSS control mode is improved.