Based on energy conservation law, and combined with system dynamics and hydromechanics, the mathematical model of the Hoffmann Hydraulic Mechanism (HHM) was established in the chamber method. Then, the RungeKutta method was selected to solve the state equations. At last, with the SIMULINK in MATLAB, the dynamic characteristic of this system was analyzed, and the oil pressure changing with time was investigated. Both are of considerable significance for the system characteristic study and design improvement.