In order to improve and guarantee the shift quality of heavyduty automatic transmission,based on the analysis of system operation principles of heavyduty automatic transmission shift hydraulic control system, the mathematical model of the hydraulic control system was established firstly, and then the simulation model was set up based on AMESim, and the change and stability of clutch hydraulic pressure were studied from those important factors including duty cycle, shift valve spring preload and the opening of shift valve. The simulation results show that the shift time is about 16 s, these factors have a great impact on the clutch hydraulic pressure when t=1~16 s, then the clutch hydraulic pressure is influenced only by the main oil change; when t=32 s,stable pressure is reached which meets the actual job requirements. At the same time the simulation results consistent with the theoretical analysis, verifying the correctness and validity of the model.It provides some reference for gear shift control, assembly and oil temperature control of automatic transmission.