To improve and maintain the operation stability of the synchronous generator is one of the basic conditions to ensure the economic and safe operation of the power system improve and maintain the stability of the synchronous generator. Excitation fault often cause unstable opertation of generator, even tripping, so we have to attach importance to it. This paper took the example of a tripping accident of XX power plant, analyse the process of tripping caused by adjusting excitation not timely after non-plan switching automatic excitation to manual excitation. And we discussed the theories of power angle characteristics and leading operation, studied on stability limit of generator opration, in order to analyse the principle of losing stablily. Meanwhile, we verified the inevitability of accident by loss-of-excitaion criterion. Besides, basing on Matlab we conducted the fault simulation, which confirmed the correctness of analysis. At last, we brought up some reasonable suggestions on avoiding this kind of accident.