: A fixed frequency hysteresis current control method based on space voltage vector is proposed for three-phase four-leg APF. In this method, a set of external hysteresis comparator is used to quickly determine the reference voltage region; the reference voltage vector space is divided into four parallelepipeds. Through a set of internal hysteresis comparator, according to the reference voltage location, three phase switches are selected appropriately to control the corresponding phase-to-phase current independently, then the fixed-frequency hysteretic control technology is realized after decoupling. The method also considers error current phase adjustment so that the error current is optimized, the control accuracy is improved, and the stability of the switching frequency is maintained. Finally, PSCAD / EMTDC power transient simulation software is uesd to validae the effectiveness of the method.