In order to study the dynamic characteristics of a closed hydraulic steering system and perform thermodynamic analysis, a simulation model and a thermohydraulic model of the hydraulic steering system were established. According to the system design requirements and field tests, the effects of hydraulic pump flow, relief valve pressure and system load on steering characteristics were studied, and the twochamber pressure of the steering hydraulic cylinder was compared and analyzed. The results show that a lower flow output can reduce hydraulic shock, an excessively high load will produce a larger hydraulic shock, and the addition of an accumulator can greatly improve the stability of the hydraulic cylinder’〖KG-*2〗s working pressure. Based on the established thermohydraulic model, the temperature rise process of the system was simulated and analyzed. The results show that the pressure setting of the relief valve has a greater effect on the temperature rise of the hydraulic cylinder, and appropriate overflow pressure should be set according to the actual situation of the load; increasing the load can cause the temperature rise of the hydraulic oil, resulting in overflow loss, increased leakage in the hydraulic cylinder and increased friction in the pipeline, so in practice, the system should be prevented from operating under extreme load conditions. Through the field test, the system parameters were adjusted again, the dynamic characteristics of the hydraulic system were improved, and the oil temperature was dropped significantly. The results provide reference for the dynamic characteristics and thermodynamic design of the closed hydraulic system.