The invention belongs to the technical field of power electronics, and specifically relates to an improved three-phase four-bridge arm topology for realizing active power decoupling used in a system containing a single/three-phase unbalanced load or source. The improved three-phase four-bridge arm topology comprises eight switch tubes S1, S2, S3, S4, S5, S6, S7, S8, four inductors La, Lb, Lc and Ln and five capacitors Ca, Cb, Cc, Cn, Cdc. When the single/three-phase unbalanced load or source exists, a fourth bridge arm is controlled, a passage is provided for a zero-sequence component by usinga center line, and then three-phase phase voltage balance is realized, and high-quality power supply is provided for the load; secondary impulse power on an alternating current side is provided by the capacitors on the alternating current side through active power decoupling control, a direct current side just needs to provide the average power component on the alternating current side, on the basis of without adding switch devices, a secondary impulse power decoupling purpose is realized, the tolerance of supporting capacitors on the direct current side can be reduced effectively, and the influence of secondary impulse power on direct current source or front system control performance is avoided.