The characteristics of automatic transmission shifting process involving multi-group clutches were studied and it was pointed out that the shifting process was a hybrid system including a continuous system and a discrete system. A hybrid modeling method, based on the continuous system under bond graph theory and the discrete system under finite state machine (FSM), was developed. A model for shifting process of hydro-mechanical transmission (HMT) equipped in the Dongfanghong 1302R rubber crawler tractor was established based on the platforms of Matlab／Simulink and Matlab／Stateflow. The simulation results showed that this hybrid system was rather effective for simulating a dynamic behavior of clutches in shifting process and its influence on the shift quality was analyzed through simulation, which served as an effective tool for the design of clutch control strategy in the shifting process.