Abstract: Speed ratio follow-up control is the core of continuously variable transmission (CVT) control system of vehicle. Theory on speed ratio control of arithmetic type hydro-mechanical CVT is studied and the speed ratio control method suitable for hydro-mechanical CVT is proposed. The composition principle of arithmetic type hydro-mechanical CVT is analyzed. The general speed ratio expression of arithmetic type hydro-mechanical CVT is deduced, and the range composition mode of arithmetic CVT is obtained. Referring to the specific structure of research object, the speed relationship among the actuators during range shift is researched and the range shift condition is found. By establishing the design diagram of speed ratio control system, the transfer function of controller is analyzed and the designing method of control algorithm is given. The tests of speed ratio follow-up control are carried out on the two-range arithmetic type hydro-mechanical CVT. Test result shows that through the control of the speed ratio of hydro-mechanical CVT, target speed ratio follow-up can be realized. There is no steady-state error in step speed ratio follow-up，but there is steady-state error in slope speed ratio follow-up

Key words: Hydro-mechanical continuously variable transmission, Shift, Speed ratio control