Abstract: The designing of speed ratio follow-up control system of hydro-mechanical transmission is given. The speed ratio characteristics of geometric type hydro-mechanical transmission is analyzed, and the model of speed adjusting system of hydro-mechanical transmission is built up. By analyzing the frequency behavior, the model of hydro-mechanical transmission is simplified and based on it, the control algorithm is designed. Finally, the speed ratio follow-up control and the performance of engine speed adjustment with speed ratio follow-up system were tested. Test results showed that there does not exist steady-state error when following the step speed ratio, but steady-state error do exist when following ramp speed ratio, and by following the speed ratio of hydro-mechanical transmission, the desired speed of engine can be achieved.