Hydraulic power assist system is a hydraulic regeneration system, which can improve fuel economy significantly coupled to a conventional bus operating in urban traffic. This paper presents a new hydraulic power assist system-HPAS. For the special configuration and control strategy, HPAS can be easily installed in an old bus as well as a new one. In this paper, more attention is paid to the drive performance of HPAS, and the effects of the component parameters on the system are simulated in AMESim. Experimental results show that HPAS satisfies the vehicle acceleration requirement and can reduce 25 percent of the fuel consumption.