Abstract: The paper proposed a scheme of using AC chopper device to compensate three phase unbalanced system dynamically and gave the device structure. The simulation result verified that the AC chopper device can adjust output voltage by changing duty ratio, and then adjust capacity value of capacitive load, which achieves dynamic reactive power regulation of unbalanced load. It also simulated application of the AC chopper device in reactive power compensation of three-phase four-wire unbalanced system by use of Matlab software, and the result showed that the AC chopper device can continuously and adjustably compensate unbalance current and pours no harmonic current into power grid.