This paper briefly introduces the composition, working principle and characteristics of a new transmission system——hydrostatic transmission, with emphasis on the matching between engine, accumulator and pump. Through mathematical model the relationship between parameters: such•as polytropic exponent, charge volume, effective volume and pressure ratio of accumulator, and their influence on energy density and braking performances of vehicle was analyzed in detail. The results can be used to design the hydrostatic transmission system and select operating parameters.