A new type of hydraulic hybrid system that combines OP2S engine and hydraulic plunger pump was designed and it had a capacity for outputting mechanical energy or hydraulic energy in vehicle propulsion according to different driving conditions though it cancels the power divider in conventional hybrid vehicles. After analyzing the operating characteristics of the new type of hydraulic hybrid vehicle, an AMESim simulation model was established to study the effects of critical switch vehicle speed on the fuel consumption and brake energy recovery. The results show that the hydraulic hybrid vehicle can achieve speed control requirements of the driver and recycle the brake energy when decelerating in an NEDC driving cycle. Finally an optimized critical switch vehicle speed of 25 km/h is obtained to make a compromise between high brake energy recycle and low fuel consumption.