In order to address motion disturbance in the electro-hydraulic servo loading system and effectively mitigate the impact of uncertainty on system performance, a proposed solution is to implement an independent load port pressure control system. By analyzing the different states of the servo valve and hydraulic cylinder chamber pressure, a nonlinear variable structure state equation suitable for loading conditions is derived. Considering the significant motion disturbances, an active disturbance rejection control approach is adopted to construct an expanded state observer and local feedback, enabling the system to exhibit quasi-integral characteristics and possess strong disturbance resistance, when the frequency of the disturbance signal is 8 Hz, the system pressure still maintains the integral characteristic. Based on this, a closed loop can be formed to achieve time-variable capacity and effectively control cavity pressure. However, due to the structure switching and pressure gain in the model, the system response may experience some buffeting caused by disturbances, particularly under severe disturbance conditions. During step commands, the anti-disturbance frequencies under auto-disturbance rejection control reach 8 Hz, while the PID is only 1 Hz. Furthermore, simulation results indicate that the random noise of the sensor has a limited impact on the system response, highlighting the filtering effect of the extended state observer. These findings provide a foundation for future advancements in coordinated pressure control of each load port of the electro-hydraulic servo system.