Abstract: Abstract: Hydro-mechanical variable transmission (HMT) is a kind of double power flow transmission system constituted by hydraulic branch and mechanical branch in parallel. HMT has the ability to realize high-power CVT and high transmission efficiency, and is suitable for high-power automobiles. In general power shift process, there are load reversal in hydraulic transmission unit, power flow reversal in hydraulic branch, and function interchange in hydraulic components. At the same time, the speed of fixed displacement hydraulic component changes abruptly, and there is a short time power interruption. To solve the problems of speed fluctuation and power interruption in power shift, based on the five-stage full power shift method by overlapping the double clutches, the displacement regulating law of variable displacement hydraulic component is studied in this paper. An arithmetic type two-range HMT is taken as the research object, and the pressure responses to different displacement regulation are analyzed. The torque characteristic equation in full power shift is derived. The cavity model of closed hydraulic circuit is established, in which the closed hydraulic circuit is simplified to two cavities. Considering the influence of the volumetric efficiency of the hydraulic transmission unit, the mathematical model of the displacement ratio regulation of variable displacement hydraulic component is derived. The step change and linear change of displacement ratio are adopted respectively, and the pressure response is obtained by simulation. The results show that the regulation law of displacement ratio has a great influence on the time of pressure building-up and pressure fluctuation. The step change of displacement ratio can effectively reduce the time of pressure building-up, and there is no obvious increase of pressure fluctuation compared with that of the linear change. The power shift time is 0.93 s and pressure fluctuation is 0.64 MPa. In order to reduce the power shift time, the step change of displacement ratio could be adopted. Through the displacement regulation characteristic test of HMT in full power shift process, the pressure response is obtained. The results show that the simulation results of displacement ratio change are in accordance with the test results, and the maximum deviation is 8.93% under the same working conditions. Based on the state parameters of current range and the target range pressure, the mathematical model of the displacement ratio regulation proposed in this paper can predict the displacement ratio target value, and accurately describe the pressure interchange between the high and low pressure circuits. During the double clutches overlapping, the displacement ratio is adjusted to the target value. The pressure interchange between the high and low pressure circuits can be completed in the full power shift process, and the torque is transferred from current clutch to target clutch. The output speed remains unchanged, and the output torque is continuous. When the output torque after speed increaser is 100 and 150 N·m, the power shift time is 1.00 and 1.10 s respectively, it's the maximum fluctuation is 6.80 and 6.84 N·m respectively. The problems such as speed fluctuation and power interruption in the shift process are solved, this study provides a reference for the realization of HMT full power shift control and subsequent research.