The light unmanned platform can not achieve the expected rapid decent function by selfweight. In order to explore the failure mechanism and cause, taking a light unmanned platform as the research object, the vehicle posture descent oil return circuit simulation model was established based on FLUENT fluid simulation software, and the simulation analysis was carried out. The research shows that the slow decline of the vehicle posture is due to the high back pressure at the end of the oilgas spring of the vehicle posture return circuit,and the formation of the high back pressure is caused by the series connection of the main overflow oil circuit and the vehicle posture descent oil return circuit and the joint action. According to the results of simulation analysis, the hydraulic system return circuit of the vehicle posture of the platform was optimized. The main overflow oil line and the return oil circuit were disconnected and diverted back to the fuel tank separately, which solves the failure of the vehicle posture decline slowly. The results of simulation analysis were verified by the vehicle test.