Hydro-mechanical transmission (HMT) is one type of mechanical-hydraulic stepless transmission. There is a better tradeoff between the stepless speed regulation of hydraulic and efficient speed change of mechanical transmission. Clutch control is closely related to the appropriate timing of engagement and separation, due to the great influence of disturbance on the quality of the shifting process. It is necessary to clarify the shifting clutch for the higher quality of HMT during shifting. However, a strong nonlinear system is coupled with the HMT under modeling error and external load disturbance. In this study, the shifting clutch pressure control was proposed using disturbance feedforward compensation. The dynamical model of shifting and the mathematical model of the clutch in the sliding were also established using the composition and working principle of HMT. The optimal linear-quadratic model was achieved, where the jerk, sliding power, and shifting time were taken as the evaluation indexes during shifting, while the performance index of the quadratic function was the state control variable. The total disturbance during shifting was also estimated to rapidly respond to the disturbance using the first-order disturbance observer. The compensation gain of disturbance was introduced into the feedforward term of the controller, in order to realize the disturbance feedforward compensation. After that, the feedforward compensation gain was obtained, according to the disturbance estimate $\hat d $ and compensation coefficient k_d. Then, the optimal controller was designed to suppress the disturbance of the shifting process for the optimization of the quadratic index. The functional weight coefficients were set as q=1, and r=0.1, the equivalent moment of the inertia of the HMT input shaft, clutch C2 driven disc, quantitative motor output shaft, and HMT output shaft were set as 0.02, 0.04, 0.05, and 1 kg·m², respectively, while the equivalent damping coefficients were set as 0.015, 0.2, 0.018, and 0.016 N·m·rad/s, respectively. The co-simulation of HMT during shifting with disturbance was carried out in the AMEsim and Simulink software. The coefficients, k_x, k_h, and k_d of the optimal control variables were calculated by the simulation model in real time. The simulation indicated that the improved control system reduced the maximum disturbance by 48.9%, the jerk by 27.8%, the sliding friction work by 29.6%, and the shifting time by 15.3%, compared with the control without disturbance feedforward compensation. Better control was achieved in the time-varying disturbance of the shifting process. The test of the hydraulic mechanical transmission was also carried out on the test bench, in order to verify the effectiveness of the control system for the better quality of HMT during shifting. Among them, the output load torque and input speed were set as 1 000 N·m, and 1 500 r/min, respectively. Test results show that there was a consistent trend of experiment and simulation, although some error was under control. Consequently, the disturbance feedforward compensation greatly reduced the maximum jerk and shifting time, while suppressing the time-varying disturbance during shifting. The disturbance of the shifting was effectively estimated to better improve the shift quality of HMT under the disturbance working condition. The findings can provide a strong reference for the engineering application of hydraulic mechanical transmission. transmission.