The invention discloses a locking method of preventing generator stator grounding protection misoperation caused by a transmission voltage. For a grounding set via an arc extinguishing coil, the method performs transmission voltage identification and locking according to the difference between a zero sequence voltage frequency and a system power frequency, and the delay reset after the disappearance of a system side zero sequence voltage 3U0, and locks stator grounding protection within set delay time after a system grounding fault is removed, and when the zero sequence voltage frequency is vastly different from 50 Hz, otherwise, the method starts stator grounding protection. Compared with a current method in which a set protective device avoids misoperation only by increasing protection fixed values, the method of the invention can effectively avoid stator grounding protection misoperation when a transmission voltage enters a free oscillation process after a system fault is removed, require no cooperation between the protection fixed values and the amplitude and time of the transmission voltage, ensure the rapidity and sensitivity of stator grounding protection motion, reduce set shutdown caused by misoperation prevention, and effectively avoid electric power plant economic benefit and social benefit loss.