Abstract: With the increase of installed capacity and penetration rate of renewable energy, the power grid gradually exhibits the characteristics of weakness with large equivalent grid impedance. Proportional-integral（PI） parameters of grid-connected inverter designed under the conventional strong grid often cause system resonance or other instability problems. To optimize the design of PI parameters for grid-connected inverter in a weak grid, the conventional method requires approximate processing or repeated trial and error, and cannot combine multiple performance indices for parameter setting. So, the stability region of PI parameters for the grid-connected inverter in a weak grid is obtained based on the D-partition method, which satisfies the multi-performance indices,such as phase margin, amplitude margin, current-loop bandwidth and short-circuit ratio, etc. And it can be visualized by graphics,so as to quickly and accurately obtain PI parameters of weak grid, avoiding repeated trial and error. Finally, it is concluded that even the bandwidth of phase-locked loop is set very low, the PI parameters of the grid-connected inverter need to be reasonably designed and selected to make the inverter operate stably and efficiently. The validity of this design method and conclusion is verified by 30 k W experimental platform.