A common-mode EMI model of a single phase flyback switched mode power supply is built after detailed analysis about EMI sources, winding voltage and shielding roles of a transformer. Its effect is validated by experiments. Based on the model, the relationships about EMI sources, coupling capacitors and common mode EMI emission are expatiated. In order to restrain EMI produced by high frequency diode at output stage of power supply, a common mode transformer method and a compensation winding method are proposed and their effects are validated of experiments. The influence on common mode EMI by a synchronous rectifier is analyzed too.