According to the working mechanism of a wheel loader,a hydraulic mechanical coupling model for the working device was established with AMESim. Simulation about the pressure and flow rate of the system was carried out under some typical working processes. On this basis, a new load-sensing hydraulic system was designed for the working device. And compared studies were made between the fixed displacement pump system and the load-sensing hydraulic system. The results show that the improved load sensing hydraulic system can better match the pump under the demand of complex conditions and presents well in energy saving and control characteristics. The way adopted can also be used in design and analysis of some other hydraulic-mechanical coupling mechanisms.