Low short circuit capacity and high grid impedance are the main electrical characteristics of weak grid, and there are some stability and power quality problems with a large number of PV power generation systems incorporated into weak grid. The increasing penetration of PV power generation easily leads to voltage rise at the end of grid, and it affects the ability of weak grid to accept PV power capacity. This paper analyzes the impact of the line impedance, X/R ratio and power factor of PV inverter on voltage deviation, proposes an adaptive control strategy to enhance penetration of PV power generation in weak grid. This method determines the optimal power factor of different grid-connected PV inverter according to the grid X/R values, and controls the output active power and reactive power to improve PV penetration under weak grid condition. Simulation results demonstrate the effectiveness of the proposed method.