In the system of renewable energy generation connectingto the power grid through an inverter, the stability of the grid-connected inverter will decrease as the short-circuit ratio of interconnection point decreases. At present, academics have done a lot of researches about this issue. However, most of them are based on the linearized state-space model and analyzed with related control theories which have a lack of mechanism analysis. This paper uses a quasi-steady state model that ignores the response time of the current loop, and studies the static stability of phase-locked loop in the grid-connected inverter. A static stability criterion similar to the traditional one-machine infinite bus system is obtained, which can intuitively draw the power limit of grid-connected inverter under the constraint of synchronization. It is pointed out that when the grid inverter exceeds this power limit, it will lose stability in the form of overvoltage. The simulation analysis is carried out with the detailed model of the grid-connected inverter to verify the correctness of the proposed static stability criterion. This work is supported by Jiangsu Science and Technology Department (No. BZ2015010).