Tradational circuit method was limited in quantitative analysis of the effects of rotor axial vent hole size on motor parameters. Based on a highvoltgae (HV) asynchronous motor with output of 400 kW and voltage of 6 000 V, a fieldcircuit coupled timestepping FEM model of 2D transient electromagnetic field was established. With the variation of rotor axial vent hole size and topological structure, the influence on electromagnetic parameters such as airgap flux density,losses, and radial electromagnetic force, was calculated and analyzed. Furthermore, the effect of rotor axial vent hole size and topology structure on starting and operating performance of the motor was also investigated. Finally, by comparing FEM simulation with experimental results, the correctness of this method was verified. A theoretical basis for the design and optimization of rotor axial vent hole size and topological structure was provided.