Abstract: The input power of the conventional constant current charging power supply increases with the increase of output voltage, and the input power drops to 0 immediately at the end of the charging, indicating that the conventional CCPS not only needs 2 times power of mean value from grid, but also causes the fluctuation of the grid voltage. Especially when the repetition of high power system is close to the grid frequency, it may cause the oscillation of the power grid, reduce its reliability and have influence on other equipment connected to the same grid. A topology with buffer energy storage is proposed, which keeps the input power of CCPS stable during the charging process. The stable value equals to the average power of whole charging process. The characteristics of series resonant circuit in discontinuous mode are fully used, there is no need of auxiliary converter, only with control of bidirectional switches. The buffer capacitor can be charged in the early stage gradually, and release the energy to the resonant circuit gradually in the later stage. At the starting and ending of charging, the energy in buffer capacitor keeps the same. The proposed topology is added to the DC-link based CCPS. The analysis and deduced process of control parameter are given. The experiment is carried out on a CCPS with 8 A output current and 5 kV output voltage. The experiment results show that at the same charging speed, the DC bus current is stable and is about 1/2 of the maximum value in conventional CCPS.