This paper introduces the process of checking, analyzing and dealing with the causes of maloperation of the first set protection of 500 kV reactor in a substation.By checking the secondary circuit of 500 kV reactor protection for power outage, it is analyzed that the main cause of reactor protection malfunction is that the cable core of the current circuit of the first set protection is damaged at the secondary junction box of TA of the high voltage bushing, which causes the induction current to enter in series, and causes the abnormal sampling of the first set of protection B phase, and causes the protection outlet action.Aiming at the existing problems, we give the corresponding treatment methods and preventive measures.