The bifurcation of three-phase grid-connected inverters based on quasi Proportional-Resonant (PR) with LCL filter is analyzed. The discrete mathematical model of the three-phase inverters with sinusoidal pulse width modulation is established in two-phase stationary coordinate, and the nonlinear behavior on the fast time scale is analyzed. The stable boundary of the model under different parameters is predicted by the bifurcation diagram and Jacobian matrix, and the stable region of the three-phase inverter is obtained. The correctness of the theoretical results is verified by Matlab/Simulink. The conclusions of this paper provide reference for the analysis, design and debug of three-phase inverter based on the quasi PR regulator. This work is supported by Natural Science Foundation of Gansu Province (No. 1610RJZA051).