Abstract: Switching power supply，which works on switch state，is a tough electromagnetic interference source．Electromagnetic interference peak can be better suppressed by distributing the energy focused on the switching frequency and its harmonious frequencies to more frequencies．To achieve the requirement of switching power supply application，the relationship of output average frequency and input，and the mechanism of electromagnetic interference suppression are given in the further analysis of the sigma-delta modulation．To overcome the deficiencies of the sigma-delta modulation used in switching power supply，the improved sigma-delta modulation is proposed．The 〖WTBX〗m〖WTBZ〗 and inverse 〖WTBX〗m〖WTBZ〗 pseudo-random sequence are used for the realization of the random frequency generator and the random noise generator in improved sigma-delta modulation．Experiment results indicate that the electromagnetic interference peak of improved sigma-delta modulation is 10 dB lower than sigma-delta modulation，and improved sigma-delta modulation has better electromagnetic interference peak suppress effect．