The four-terminal three-phase-two-phase matrix converter(3-2MC) with compensation units can effectively improve the input performance of the matrix converter with two-phase independent load. Under the existing modulation method, the voltage gain of the four-terminal output 3-2MC is lower than that of the three-terminal topology, and the voltage gain is an expression related to multiple variables. In order to solve these problems, the article proposes a switch combination optimization method for cooperative modulation on the output side. This method performs cooperative modulation on the output side and the compensation side of the same polarity in unit modulation cycle time to improve the effective vector utilization. By theoretical derivation and analysis, compared with the existing modulation methods, the proposed method can improve the voltage gain of the four-terminal output 3-2MC to the level of the conventional three-terminal output 3-2 MC, and make the voltage gain of the four-terminal output 3-2MC be a function expression only related to the output asymmetry. The simulation results show that the optimal switch combination method based on the output side cooperative modulation can not only realize the decoupling of input and output power, but also effectively expand the linear regulation range of fore-terminal output 3-2 MC and make the system still have good input and output performance.