The landing gear ground loading tests is a vital part before flight. As the motion of landing gear is three-dimensional motion,the traditional loading method cannot effectively simulate the pneumatic load on the ground. In order to simulate the load in the process of retraction/extension, a loading simulation system is proposed, including retractile system, position follow-up system and force servo loading system. This paper analyzes retraction/extension mechanism of the landing gear and builds up its mathematical model. A co-simulation model is developed in which the hydraulic system model and control system are built in software AMESim, the mechanical system of test platform model is built in software ADAMS. The results of the simulation confirm that the loading simulation system is feasible and provide the basis for building hardware platform.