Large wide body commercial aircraft usually used the two side stays as the support and retraction & extension structure for main landing gear. The dual side stay landing gear has four attachment points with the airframe, which is a typical statically indeterminate mechanism. Compared with statically determinate structure, the dual side stay gear has to deal with the problem of extension synchronization to make a reliable extension. In this paper, taking the prototype of a dual side stay landing gear as the research object, using the rigid-flexible coupling dynamics simulation to analyse the extension and down locking performance. Focusing on the affect analysis of heading aerodynamic load and wing deformation to gear down locking performance. The lab test is also conducted to verify simulation result. The research concluded that heading aerodynamic loads can cause delays in locking the aft side stay, and with excessive heading loads even preventing the landing gear from achieving the down locking. Wing deformation can lead to the movement of the landing gear attachment points, the impact to down locking is related to the relative distance changes between after and forward side stay attachment points. The relative distance increase can help this side stay much easier to down locking.