There is a broad prospect for Alternating Current Excitation Generator (ACEG) in improving the stabilities of extra-high voltage power system and in constant frequency generation with variable speed, by using the three-phase alternating current excitation and realizing independent regulation of active power, reactive power and rotate speed. Rotor one point to ground fault is a common kind of faults of ACEG. This paper uses 100% rotor earth fault protection with DC voltage injection and analyses the influence of DC voltage injection on AC-excited system. By adopting DC voltage injection, the result of this scheme is free of operation state of the generator. The scheme can protect the 100% coverage generator rotor one point earth fault and the sensitivity has no relationship with the fault location.