The working medium’s influence on performance under low temperature environment was researched,taking an aircraft hydraulic system as object. The hydraulic system model was established based on AMESim. The influence of the hydraulic oil viscosity on the performance of the hydraulic system under different temperature was simulated. Through simulation analysis and optimization,system improvement scheme was put forward and system performance tests were carried out. The test results verify the validity of the simulation model and the feasibility of the improved scheme.