Abstract: In the three-phase four-wire power supply system based on inverters, the neutral line is usually used to provide a path for the zero sequence current. In a three-phase four-wire T-type three-level inverter with balanced leg, the midpoint current of the three-level phase leg and the load neutral line current can be bypassed by the balanced leg, so that the part flowing into the DC bus capacitor is reduced, independent control of midpoint voltage is realized, and the ability of the system with unbalanced load is enhanced. To avoid excessive loss caused by all the neutral line current and midpoint current flowing into the balance leg, it is necessary to adjust the balance leg branch current and the DC bus capacitor branch current to be proportionally distributed to achieve the balanced bridge arm loss optimization. In view of the problem that changes in load power and unbalance degree are not considered in the design process of optimal distribution ratio in existing studies, an adaptive loss optimization method is proposed, which adaptively adjusts the distribution ratio according to the amplitude of midpoint voltage ripple, and ensures the minimum branch current of the balance bridge arm under the premise of stable operation of the system, so as to obtain the optimal distribution ratio under different working conditions. The design step of key parameters in the proposed method is also given. Finally, a 3 kV·A three-phase four-wire T-type three-level inverter experiment platform with balanced leg is built, and the experimental results verify the feasibility and superiority of the proposed method.