Abstract: This paper introduces a topology structure of Buck-Boost converter based on AC-Link series resonant. The state-plane analysis method is used to analyze the working process of series resonant circuit in Buck mode and Boost mode. This method is relatively direct, accurate and simple compared with the simple basic wave equal effect analysis method. A detailed derivation is given and a control algorithm is proposed. The simulation model of Matlab/Simulink is set up. The simulation results show that the control algorithm can realize the function of Buck and Boost, and low current harmonic content. And the converter can adjust the output voltage in a wide range.