Compensation capacity plays an important role in the operation performance of shunt active power filter (SAPF). The determinants of SAPF compensation capacity are analyzed by Thevenin equivalent model of grid and load. The shortcoming of compensation capacity calculation method according to load current before compensated is pointed out. Furthermore, a compensation capacity prediction method of SAPF based on harmonic source injection which uses small capacity harmonic source is proposed. Characteristic coefficient of harmonic source and compensation capacity magnification are also given, which provides direction to type and capacity selection of SAPF. Ultimately, availability of the method is testified through simulation. Compared with the traditional method, this method is more close to the real capacity of APF.