This paper briefly introduces the structure and main function of a mild hybrid system based on ISA. The components of the whole powertrain system include ISA,engine and battery. Transmission is choosed according to its typical configuration and functions. In addition,the specific parameters of power and torque of ISA,engine power,the number and capacity of battery,the gear ratio of gearbox are determined according to the required performance and fuel economy. In order to achieve the design target, the Genichi Taguchi method is used to optimize the gear ratio. At the same tine, the simulation of ISA mild hybrid vehicle is carried out and compared with conventional vehicle by using the ADVISOR software.