As we all know, with the continuous improvement of people's living standards and the rapid development of industry, the power system has higher and higher requirements for power quality. Therefore, in recent years, engineers continue to develop new reactive power compensation devices to compensate a large amount of reactive power required by the load. Among them, cascaded static var generator (SVG) is generally used as reactive power compensation equipment in distribution network. Although cascaded SVG has low switching loss, fast response speed and high reactive power compensation efficiency, large electrolytic capacitors need to be paralleled in DC side to filter low order harmonics. In this paper, an active power decoupling technology is proposed. An active power decoupling circuit is paralleled on the DC side of each submodule of SVG. That is to say, it can effectively reduce the required capacitance on the DC side while keeping the voltage fluctuation on the DC side unchanged, thus effectively reducing the volume of SVG. This paper analyzes the relationship between DC side capacitance and voltage fluctuation from a theoretical point of view, and verifies the theoretical analysis results by using the simulation platform. It is found that the simulation can well verify the theoretical analysis results.