The maximum explosion pressure appears in the connecting cavity connected with the main cavity, which first decreases and then increases instantaneously, when the determination of explosion pressure test is carried out in YB3-315L2-4 (660/1140) flameproof three-phase asynchronous motor. By the simulation and theoretical analysis of the motor model, it is proved that the connection cavity changes the propagation path of pressure wave, which results in the displacement of the explosion compression point, and the stronger the compression effect is, the higher the explosion pressure rise rate is, and the greater the explosion pressure is. Finally, some suggestions for flameproof optimization design are put forward.