The scale of photovoltaic (PV) system tends to be large but the ability of power transmission has not been fully improved. When the power generated by PV array is imbalanced with the power output by inverter, system becomes unstable. The instability caused by power imbalance in large PV system is rarely discussed but causes big loss once it happens. Even energy storage and reactive power compensation can help to balance the power, there is a lack of a reliable control strategy for solving this problem. This paper analyzes the stability problem caused by power imbalance in large-scale PV system, and aims to propose an optimization control to improve system stability. The output ability of large PV system is studied, and the influence of grid impedance and power factor on inverter output power is analyzed. One method based on real-time inverter control is investigated to calculate the maximum output power of inverter system, and the optimization control is proposed and implemented based on maximum power point tracking (MPPT). Finally, the correctness of the theories is validated by simulations and experiments.