Abstract: In recent years, the malfunction of protective relaying devices occurs from time to time due to the fault of protection devices, the error of setting values, incorrect judgment logic, incorrect secondary wiring and other reasons, which brings about great challenges to the safe and stable operation of the power system. Based on the analysis of a 220 kV bus failure protection malfunction accident, various factors that might cause the accident are discussed and analyzed, and finally it is determined to be the damage of startup exit relay at the CSC103B line protection exit board which should have been started in case of protection failure, resulting in long-term closure of the C-phase failure startup panel point which makes the C-phase failure startup circuit is switched on instantaneously when the C-phase failure occurs.