Abstract: To decrease the range-shifting impact on hydro-mechanical transmission (HMT),theoretical analysis and test-bed experiments of the range-shifting process on one kind of HMT had been done. It can be concluded that the mismatch of the rotating speeds of the three basic elements of the planetary-gear system is the essential factor leading to the range-shifting impact. The existence of the synchronized range-shifting gear ratio was proven and it is only determined by the mechanical structure of the HMT. The variation of the motor speed and output speed during the under-synchronized and over-synchronized range-shifting were analyzed and investigated with test bench experiments. The results show that, the volumetric efficiency of the hydraulic system will cause a sudden shift of the motor speed and further the impact on the planetary-gear system, the distinctness of the engagement and disengagement characteristics of the brakes is one of the reasons resulting in the interrupt of the power transfer. While the two brakes use a common oil source, the mutual influence of the manipulating hydraulic pressure may cause the power transfer interrupt and torque impact.