With advantages of ride comfort and fuel economy, the hydro-mechanical continuously variable transmission (HMCVT) technology has been widely used. A 176kW tractor HMCVT sketch with single row planetary gear structure and double-session was introduced as a mathematical model to study system characteristics. The theoretical model of transmission ratio distribution was established, and the possible power flow directions for the HMCVT were researched with the changing of pump displacement ratio. For display of steady characteristics and prediction of output efficiency, efficiency expression of the HMCVT was obtained by study of torque ratios of the epicyclic gear train (EGT) links, and by study of torque ratio of the hydrostatic transmission system (HST). Through solving the equations of flow continuity of the HST and torque balance of transmission input shaft, the HST and the EGT systems were connected organically, a numerical method for HMCVT system efficiency was introduced. According to the shift strategy, the HMCVT physical prototype was tested under 10 working conditions and the traction operation of the tractor was simulated. The results of bench test indicated that the transmission efficiency of the system was higher than 0.85, except working conditions 1 and 2. Comparing with the bench test results, the simulation reflected the steady state characteristics of the HMCVT, efficiency error was about 4.7% under the minimum displacement ratio condition, and errors were less than 2% under other conditions. This method can have guiding significance in the stage of engineering research and development.