Currently, vacuum type generator outlet circuit breaker dominates the large-capacity generator circuit breaker. However, due to the large short-circuit current at the generator outlet end, the generator outlet circuit breaker needs to bear a great short-circuit electromotive force, thus affecting its dynamic stability. In this paper, COMSOL is used to construct a model of a six vacuum break parallel type outlet circuit breaker for the protection of a megawatt-level generator. Based on Maxwell's electromagnetic theory, the transient calculation of three-phase short-circuit electromotive force under different phases is carried out for the generator outlet vacuum circuit breaker, and the great values of short-circuit electromotive force under different phases are obtained for each phase. The results show that the main component of the short-circuit electromotive force is the electromotive force in the y-direction. And based on the maximum transient electromotive force, the influence of the material on the short-circuit electromotive force is studied, which provides theoretical support for the analysis of the short-circuit electromotive force of the generator outlet circuit breaker and the optimized design of the shell.