To achieve automatic shift function of a new type of modular transmission, a hydraulic shift system was designed for the prototype and the simulation model of shift process was built. A prototype with two speed was analyzed based on the principle of rigid body dynamics, a model including the driveline and hydraulic cylinder was established in MATLAB/Simulink. A simplified hydraulic shift system was designed with overflow pressure regulation. By using output torque change rate and input minimum speed as the evaluation index, the influence of parameters including filling flow and spring preload on the prototype's shift quality was studied. The test achieved the shift of prototype and verified the reliability of model. According to the simulation, the filling flow determined the velocity of part and the rising speed of inner pressure of cylinder, which affected the shift speed and braking. Increasing filling flow could effectively eliminate braking, but cause increase of shift impact; decreasing the preload of spring could reduce the shift impact.