Abstract: To analyze and improve the stability of photovoltaic grid-connected system, a two-stage three-phase photovoltaic grid-connected system small-signal model including phase-locked loop (PLL) is established. The eigenvalue method is used to analyze the influence of power grid strength, maximum power point tracking (MPPT) control parameters and PLL control parameters on system stability. The dominant oscillation mode under weak grid condition is directly related to the control parameters of PLL. Based on the interaction between PLL and inner current loop, an additional control method is proposed. The output frequency from the PLL is introduced to the inner current loop through the additional control coefficient. From eigenvalue analysis and time domain simulation, it is proved that this method can solve the system stability problem caused by improper setting of PLL control parameters. Then the lower limit values of the additional control coefficient under different PLL gains are given.