Abstract: Some general knowledge of EMI is introduced in this paper.The basic principle of causing differential mode and common modecurrent related conducted emissions of EMI in the switch mode powersupply is discussed in detail through the relevant circuit theories and thesketch diagrams. The design of the cancellation inside the structure of thetransformer is also presented and explained by the sketch diagram. Themethods for reducing radiated emissions of EMI are described as well too.In the end the design for common mode inductor is offered.