All the parametric formulas in the discontinuous current mode of CCPS are given in this paper. Due to the leakage inductance and distributed capacitance existing in the high voltage high frequency transformer, the charging current decreases as the load voltage increase. To solve the issue, a dual-mode modulation strategy between pulse frequency and pulse width modulation is taken. Finally, a capacitor charging power supply of 100V input and 1600V output is designed. The experimental results confirm the correctness of the theoretical analysis and modulation strategy.