The grid-connected photovoltaic system will become unstable when the power grid impedance exists. In order to solve this problem, this paper analyzes the weak grid impedance impact on the stability of grid-connected photovoltaic system, reveals the reasons of instability of control system when the power grid impedance exists. At the same time put forward a control parameter tuning method according to the grid impledance, this method solves the problems of control instability. The design example and simulation have been verified the practicability and availability of the theoretical analysis and proposed control parameter tuning.