By adding a hydraulic propulsion system made up of pumps and motors primarily in a traditional heavy vehicle, it is changed to a four-wheel driven vehicle from a two-wheel driven vehicle. The principle of the system configuration and working modes were analyzed. The system dynamics equations were established. The whole vehicle system was modeled by a co-simulation method with Matlab/Simulink and AMESim software. The vehicle’s traction force and climbing capacity was simulated and compared. The results showed that after using the hydraulic propulsion system, the vehicle traction force was increased by 13.4% and the climbing capacity was improved by 14.4%, which means the vehicle passability was also improved significantly.