DC bus midpoint voltage excursion of four-switch three-phase (FSTP) inverter leads to its output imbalance. Based on FSTP equivalent circuit analysis, it concludes that power supply impedance difference is the internal cause of FSTP output imbalance. To get a better control of FSTP inverter, according to vector synthesis equivalence principle, an improved triangular wave modulation PWM method is proposed to replace SVPWM. To solve the output imbalance problem in FSTP inverter, a voltage reference generation method with DC bias compensation is brought forward. Simulation results show that both output imbalance problems caused by DC midpoint offset and parameter difference can be effectively inhibited, and it is easy to implement.