Three-phase balance degree is an important quality index of power grid operation. Excessive three-phase imbalance will bring serious harm to power grid. To solve the problem of three-phase unbalance of low voltage power grid, this paper proposes and implements a automatic distributed photovoltaic phase-selective grid-connection system based on PLC control. The main functions include: photovoltaic automatic phase-selective grid connection, realization of three-phase unbalanced treatment of low-voltage power grid, low voltage and over voltage treatment of power grid. The main structure includes: monitoring & execution unit, logic control unit and photovoltaic power generation unit. Based on the voltage and load of each phase of the power grid, photovoltaic power generation module can realize automatic phase-selective grid-connection, so as to improve the three-phase imbalance of low-voltage power grid, solve both low voltage and over voltage problems, and realize the comprehensive control of voltage level of low-voltage power grid.