The simple dynamic model of the shift process of automatic transmission was built. The torque phase and the inertial phase of the shift process were analyzed with the model. In the torque phase, the accurate timing to switch from release clutch to the apply clutch has been controlled, which reduced the disturbance and avoided the interrupt of the torque. In the inertial phase, the pressure increasing on the apply clutch has been controlled smoothly through the real time-closed loop modulation of pulse-width modulated valve, which improved the shift quality of the automatic transmission. The recorded test data was analyzed, and the theory of shift quality improving has been proved rightly and practically.