Abstract: Hydro-mechanical continuously variable transmission (CVT) is one of the mainstream technologies for the future development of tractor transmission systems, and the efficiency characteristics of the hydraulic pump-motor are crucial for the dynamic performance of CVT. To solve the existing problems in predicting the efficiency of hydraulic pump-motors, including in defect of the analysis on the power loss between friction parts and taking much calculation time with the finite element efficiency model, a semi-empirical efficiency model was proposed based on loss mechanism for all operating conditions to evaluate exactly the efficiency of hydraulic pump-motor. Considering the nonlinear changing regularity of system parameter, some functions of the model were designed to predict the power loss, to fit the compound system parameters based on experimental data, finally to obtain a complete efficiency model for the hydraulic pump-motor. Experimental data shows that the average relative error of the model is 6.36%. The volumetric loss in high-pressure differential regions and the mechanical loss in low-pressure differential regions are the main influence on the pump-motor efficiency. Causing the difficulty in improving efficiency of hydraulic pump-motor under margin working conditions, the main reasons are the flow leakage, compressible volumetric loss and Coulomb friction loss. It is the beneficial for improving pump-motor efficiency and enhancing the overall transmission efficiency of CVT to operate the pump-motor in medium to high displacement regions and avoid low-pressure and high-speed conditions.