The clamping switch capacitor-based DC/DC (CSC) is an effective module to construct the DC transformer with the capability of DC faults clearing. However, the voltage amplitude of capacitor and AC-link will be influenced directly by the change of DC voltages in the traditional control of CSC because of the coupling effect of DC voltage and capacitor voltage, thus the loss of soft-switching, the increase of current stress and power loss may occur when the DC voltages are unmatched. In view of this issue, a capacitor voltage decoupled control strategy based on the variable duty-ratio is presented for CSC to improve the switching and efficiency performance when the DC voltage is unmatched. The proposed control strategy can guarantee the amplitude of AC voltages is matched in the full-operating-range of CSC automatically, so the efficiency of CSC can be enhanced obviously. The correctness and effectiveness of the proposed control strategy are validated by the prototype.