This paper studies an extended dual-machine system with a Doubly-Fed Induction Generator (DFIG), and examines the influence of the frequency regulation of the DFIG on the transient power angle stability of the system. The two-machine system with a DFIG is equal to the single-machine infinity system with a DFIG, taking into account four factors such as wind power ratio, wind turbine grid connection location, fault location, and load access location. The theoretical calculation of the critical clearing angle of the system is carried out using the equal area rule in two cases of before and after the DFIG participating in system frequency modulation, where the DFIG uses an equivalent model of a controlled current source. At the same time, it builds a simulation model of an extended dual-machine system with a DFIG in PSD-BPA to verify the correctness of the theoretical analysis. The results show that a DFIG participating in system frequency modulation is beneficial to the system's transient power angle stability. This work is supported by the National Natural Science Foundation of China (No. 51367009) “Research on Strategy Setting, Risk Assessment, and its Coordination Optimization of Coincide Sequence under Multi Perspectives”.