The integration of a large number of single-phase loads in distribution networks could lead to the imbalance of three-phase power flow. In order to eliminate a series of hazards caused by the unbalanced three-phase loads in distribution networks, an automatic balance control method is proposed to mitigate the degree of the three-phase unbalance of the distribution transformer. Subsequently, the structure of the commutation type unbalanced load automatic balance control system is systematically analyzed, and the commutation control flow and commutation strategy are further designed. The control flow of the circuit breaker and thyristor of the commutation unit based on the composite switch is elaborated. Finally, a three-phase unbalanced simulation model of the distribution transformer is established to verify the effectiveness of the proposed control strategy and control flow.