In order to study the conduction common-mode electromagnetic interference（ EMI） characteristics in the communication power supply, based on the analysis of the transmission mechanism of common-mode noise in the half-bridge LLC power supply, determine that the transformer is the key component that affects the common mode noise characteristics of the circuit. By analyzing the mechanism of the effective capacitance of the common-mode port of the transformer, it is pointed out that the key to changing the effective capacitance of the common-mode port is to adjust the layered structure and charge distribution of the primary and secondary windings. A method to adjust the effective capacitance by adding compensation windings between the primary and secondary windings is proposed, and the structure of the compensation winding is simulated and designed using finite element simulation software. In this way, the effective capacitance of common mode port of the planar transformer is optimized, and the common mode EMI characteristics of the planar transformer are well improved. Finally, an experimental verification was carried out on a power factor correction（PFC） +half-bridge LLC communication power supply. The test results show that the common mode noise of the prototype can be effectively reduced by optimizing the effective common mode port capacitance of planar transformer.