Aiming at the stability problem of frequency and generator angle after commutation failure in HVDC system. This paper analyzed the mechanism of commutation failure in detail based on the commutation process. Considering the moderating effect of governor on mechanical power in transient process after faults, the power angle characteristics of the generator are divided into unstable mode and stable mode based on equal area rule. The two modes are relative to the commutation failure duration and DC capacity. The influence of water hammer time constant on system response after commutation failure is analyzed in stable mode. The mechanical power control of the governor and hydraulic turbine shows a negative regulation effect for the transfer function of hydraulic turbine is non-minimum phase system. The conclusions are verified by BPA program.