On-line estimation of oil pressure for shifting actuators can be useful for the development of close-loop control of automatic transmission fitted to heavy off-road vehicle for good shift quantity under complex working condition. The unified dynamic model for a 3 degree-of-freedom planetary automatic transmission is given, and the power-on 1st-2nd up shift is considered as an example. The unified model is more efficient than conventional model for the dynamic analysis, meanwhile, it provides a computational method for the inertia of transmission in-gear. The real-time estimation of oil pressure for shifting actuators during sliding viz. torque phase and inertia phase is addressed. The changes of vehicle operating conditions is reflected in the pressure estimation, and the regulate of pressure for shifting actuators is improved. Then the up shift controlling scheme is developed and validated via a heavy off-road vehicle test bench. Experimental results show that the model-based oil pressure estimation is able to reflect the system dynamic characteristics in the changing condition, and the according control strategy can improve the shift quality and the vehicular performance effectively.