With the rapid development of China's telecommunication industry, the overall size of communication network in China has leaped to the forefront of the world. The demand of communication equipments for the power system is also getting higher and higher. In the noise interference of high frequence switching power supply there is not only harmonic conducted interference but also electromagnetic interference(EMI). Aiming at the instability of communication high frequence switching power supply, analyzes the mechanism of producing EMI in high frequence switching power supply and puts forward effective measurements to restrain EMI.