The invention provides an electric system cascading failure evaluation method for considering relay protection inherent defects. The method comprises the following steps of 1, reading data and operation referring to variables and simulation rules; 2, simulating Monte Carlo analysis and calculating the loss probability; 3, conducting visualized output displaying on a loss index. According to he electric system cascading failure evaluation method for considering the relay protection inherent defects, the loss estimation is conducted on cascading failures of the system based on a probability space technology, the obtained loss index which is quantized after estimation represents the weak link of the system, and strong support can be provided for decision making of planning personnel for system running.