The invention discloses a method of evaluating longitudinal differential protection device abnormal action probabilities caused by a communication delay error code, which belongs to the technical field of power system control. Longitudinal differential protection signal transmission delay and error code information of the longitudinal differential protection device are measured in real time; according to the delay and error code information obtained through measurement, the abnormal action probabilities of the current longitudinal differential protection device are calculated, the probabilities comprise a protection device error action probability caused by transverse asymmetric delay, a protection device failure-to-operate probability caused by longitudinal symmetric delay, a protection device failure-to-operate probability caused by an error code and a protection device failure-to-operate probability caused by a delay error code. Through measuring protection signal transmission delay and error codes in real time to analyze influences on a relay protection device by the current communication system, defects of the current relay protection device in the aspect of reliability evaluation can be remedied, and practicability is strong.