Through the analysis and comparison of field data obtained from actual measurement， the interference source properties of important relay protection's out-tripping relay and switching input optical couple that generating mal-operation are analytically demonstrated. The corresponding relationship between the safety action power of important relay protection's out-tripping relay and switching input optical couple and the length of external cable is analyzed. The results show that its safety action power not only relates to the length of external power cable， but also relates to the action time. It is proposed to add action time delay of no more than 2 ms on important switching input optical couple logic， replace the middle relay by fast middle relay with less than 2 ms action time， under the premise of the guarantee of safety action power， furthermore， to enhance the safety equipment performance of the protection device.