Significant pulsating power exists on the dc side of three-phase four-wire inverters in the case of unbalanced or nonlinear loads, which will degrade the power quality of the dc side voltage. To address this issue, this paper presents an active power decoupling control scheme for a three-level buck four-leg current source inverter (3L-Buck-4L-CSI) topology. By fully utilizing the flying capacitor converter, this scheme can effectively buffer the pulsating power into the flying capacitor without modifying any hardware circuit and suppress the pulsating input current. The experimental results validate the effectiveness of the proposed method.