Taking into account the frequency-modulation characteristics of doubly-fed induction generator(DFIG), the equivalent impedance model of DFIG for transient power angle stability analysis is established. For the extended dualmachine system with DFIG, it is equivalent to an extended single-machine infinite-bus system. Based on the equal-area criterion, the expression of the limit clearing angle about the wind power proportion and the droop coefficient in frequency-modulation control is derived. The change trend of the limit clearing angle of the system with the wind power proportion and droop coefficient of DFIG is analyzed theoretically, and the internal mechanism of the change is revealed in combination with the trend of power-angle characteristics. Considering the low-voltage ride-through characteristics and frequency-modulation control of DFIG, a detailed model of DFIG is built in PSD-BPA. The extended dual-machine system with detailed wind power model and Yunnan power grid simulation model are built to verify the theoretical analysis results. Theoretical analysis and simulation results show that DFIG participating in frequency-modulation can improve the transient power angle stability.