Abstract: This paper introduced basic structure of Switching Power Supply (SPS) block diagram and the theory of its EMI.Besides, it described that EMI for SPS is mainly about conductive interference. In order to suppress conductive EMI, this articlefirstly analyzed the common method---grounding and filtering, then the measurement technology of conductive EMI---LISN andintegrated EMI filter were analyzed and discussed. It is proposed that LISN was added to the separated network of common anddifferential mode, and designed the cascade theory of integrated EMI filter. But the theory of this paper was not verified onengineering experiment, so it is necessary to study on.