A static var generator（SVG） device based on an improved time-domain transform algorithm（TTA） is described in detail in this paper， which is applied to three-phase four-wire power system. Firstly， the characteristics of load with a three-phase four-wire power system are dealt with. Secondly， the basic operating principle of the SVG device is discussed. The improved TTA is presented after TTA based on the three-phase three-wire power system is extended further in order to detect the load current. The improved TTA has the advantages of higher accuracy and shorter response time. Finally， some experimental and running results are obtained， which show that the device has excellent performance. It can not only greatly decrease total harmonics distortion but also effectively compensate for the reactive and unbalanced current.