Permanent magnetic synchronous generator (PMSG), as the main equipment of offshore wind power generation, is an important development direction of wind power technology. However, the transient stability of PMSG decreases when it is disturbed by large disturbances. Therefore, in view of the difficulty in determining the threshold of transient stability of PMSG fans under large disturbance, this paper established a simplified mixed potential function model of PMSG fan system based on the theory of mixed potential function, and derived the criterion of transient stability of PMSG fans after large disturbance. The accuracy of the transient stability criterion was verified by the simulated grid-side current waveform and grid-side power waveform, and the correlation between the transient stability of PMSG fans and fan parameters, fault depth, fault time and wind speed were analyzed by adjusting the parameter values.