Planar PCB high-frequency transformer is the key component that affects the characteristics of conducted common mode (CM) electromagnetic interference (EMI) of switching power supply. In this paper, the calculation method of the effective capacitance of the CM port of the planar transformer is explained, and the effective capacitance of the CM port of the planar transformer is further determined by the simulation method. Then, the method of CM EMI characteristics evaluation of planar transformer insertion loss (IL) is explained and expounded, and its simulation and measurement methods are introduced. Finally, based on the analysis and research of the defects of the original transformer IL model, the high-frequency model of transformer IL is proposed, and the advantages and effectiveness of the proposed high-frequency model in the conduction frequency band are verified by the actual measurement.