Abstract: Aiming at the unstable steering of a four-wheel independent electrically driven high clearance sprayer due to the failure of the hub motor controller to respond to a large disturbance, an auxiliary steering method based on electrically controlled hydraulic pressure was proposed. Firstly, the structure and steering principle of the self-steering chassis of the four-wheel independent electrically driven highland gap sprayer were briefly introduced. Secondly, the realization method of the hydraulic assisted steering system was described, including the establishment of the steering resistance moment calculation model, and the analysis of the steering resistance moment required by the wheels under rolling and sliding conditions. The upper limit of the steering resistance moment can be calculated when the two wheels fail turning. Thirdly, based on the structural characteristics of the self-steering chassis, the kinematics model of the hydraulic auxiliary steering was established, the relevant parameters of the important components of the hydraulic system were calculated, the installation position of the hydraulic cylinder was determined, and the key components of the hydraulic system were selected. Finally, the simulation and experiment were carried out to verify the performance of coordinated control of auto steering and auxiliary steering. In the simulation test, the independent operation of self-steering, the independent operation of auxiliary steering, and the collaborative operation of the self-steering and auxiliary steering of the sprayer were simulated and adjusted to make the angle tracking trajectory of the self-steering and the auxiliary steering as consistent as possible. The simulation results showed that, self-steering alone, auxiliary steering alone, and coordinated self-steering and auxiliary steering could all track to the target angle by about 1.8 s. The response of the self-steering is relatively slow in the early stage, but it is a little faster than the auxiliary steering when tracking the target Aangle in the later stage. The auxiliary steering has a slight overshoot after tracking the target Aangle and keeps this slight error in the later stage. Since the self-steering and auxiliary work together has faster response performance than the first two separate operating systems, they are more stable when they reach the target angle. Under the working conditions of independent operation of the self-steering system and cooperative operation of the self-steering system and auxiliary steering system, the comparison tests of downhill and paddy field with a gradient of 15° were carried out at a speed of 1 m/s respectively. The test results showed that in the downhill test, the maximum tracking deviation of the independent auto-steering system operation was 6.1°, and the maximum tracking deviation of the co-operation of auto-steering and auxiliary steering was 0.9°. In the paddy field test, the maximum tracking deviation of the independent auto-steering system operation was 10.3°, and the maximum tracking deviation of the co-operation of auto-steering and auxiliary steering was 1.5°. The experimental results verify the feasibility and stability of the proposed hydraulic auxiliary steering system. The system has good test performance and can meet the actual operation requirements.