DC side capacitance is one of the key parameters for Active Power Filter(APF), appropriate capacitance directly affects APF DC voltage control effect and the cost of APF. This paper firstly revealed the conventional calculation principle of APF DC side capacitance based on the instantaneous power theory. Then for the typical nonlinear load-three-phase fully controlled rectifier bridge, analyzed the active power of rectifier bridge DC side, and then calculated the active power AC component and APF DC side capacitor charging and discharging energy. The analytical formula of the DC capacitance is derived. The formula derivation process is simple, and it has the advantage of easy to calculate and convenient for theoretical analysis. Finally, by analyzing the relationship of APF main circuit parameter design influence, the influence of DC side capacitance value on APF compensation effect is given. The validity and effectiveness of the conclusion were verified through the simulation model and the comparison with the results by other relative article.