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