The three active bridge (TAB) DC-DC converter based on phase shift plus duty cycle control strategy has the advantages of high efficiency and expandable soft switching range, but its small signal modeling process is complex and the parameter setting of closed-loop control loop is difficult. To solve the above problems, a full order continuous generalized state average modeling and PI controller design method for TAB converter under phase shift plus duty cycle control strategy is proposed in this paper. Firstly, the operation principle and Y-type equivalent structure of TAB are analyzed. Combined with the characteristics of phase shift plus duty cycle control and the equivalent method of AC square wave source, the generalized state space average model of TAB is derived. Then, based on the derived model, the transfer function from input to output is obtained, and the parameters of PI controller are designed. Finally, the correctness and effectiveness of the proposed method are verified by digital simulation and prototype tests.