Due to the rapid development of modern rapid power electronics, power system harmonics are also increasingly serious harm, in order to effectively suppress harmonics to improve power quality, active power filter APF came into being, it can be effective for reactive power and frequency, amplitude harmonic changes are real-time tracking compensation. Currently, APF power electronics technology has become a major research topic. This article describes the basic working principle of harmonic current detection method and compensation current control method, the main circuit type of active power filter.