Abstract: In order to study the dynamic characteristics of hydro-mechanical continuously variable transmission deeply, the speed governing system model of hydro-mechanical continuously variable transmission was built. The influences of oil viscosity, working volume and inertia of output port on damping ratio, rising time and setting time were analyzed, and influencing rules of those three factors on system dynamic characteristics were achieved. The analysis results show that higher oil viscosity or smaller moment of inertia results in faster response speed and larger overshoot, and smaller working volume results in faster response and smaller overshoot. Therefore, decreasing working volume of hydraulic branch is the best way to improve the performance of hydro-mechanical continuously variable transmission. In the end, the conclusion was proved by test.