In order to solve the problem of low efficiency, low function, large weight and complex circuit, a bidirectional DC-DC converter is designed to realize the high efficiency of constant current charging and constant voltage discharge for lithium batteries. The main control unit is based on the STC12C5A-60S2 microcontroller and uses a synchronous rectifier Buck-Boost circuit. The drive part controls the power MOS tube with IR2104 as the center. Constant voltage charging range 1～2 A, current step value not more than 0.1 A, current control accuracy not less than 5 %, charging efficiency greater than 95%. The constant voltage discharge voltage is 30±0.5 V, and the discharge efficiency is greater than 90%. The experimental results show that the whole design prototype has simple circuit, high charging and discharging efficiency, and no more than 500 g in weight, and has a certain practical application value.