The shape and size of EMI ferrite core affect effect of interference suppression. Generally, the larger ferrite core’s volume, the better suppression effect. Longer and thinner core has larger impedance and better effect than shorter and thicker core when the volume is unchanged, We should consider saturation problem in the case of bias current. The larger the ferrite core’s cross section, the more difficult it is to saturate, and it can bear larger bias current. In addition, the smaller the inner diameter, the better the suppression effect. In a word, the selection principle is to select EMI ferrite core as long as possible, as thick as possible and as small as the inner hole when design space is fully considered.