To solve the problem that the derailment coefficient of the rail changing robot was conservative, and the reduction rate of wheel load was used as an auxiliary criterion without unified criterion, a method of identifying the derailment of the rail changing robot by impact angle was proposed. Combined with the definition of derailment coefficient and wheel load reduction rate, the quasistatic derailment criterion of train was introduced, the quasi-static model of wheelset moment was established, the multi parameter coupling derailment discriminant of the rail changing robot was constructed, the functional relationship between different friction coefficient, different undercut angle and derailment coefficient, and the mapping relationship between undercut angle and derailment discriminant were explored. The research results verified the correctness of the theory that increasing the wheel flange angle and reducing the friction coefficient between wheel and rail were conducive to reduce the risk of derailment, and reasonably explained that when the derailment coefficient reached the derailment standard, the derailment robot did not occur, which provided a theoretical foundation for the further study of quasi-static derailment of the rail changing robot.