Abstract: Electromagnetic interference(EMI) of the harmonic frequency can be effectively reduced by frequency modulation, which is spreading single harmonic energy into a certain frequency band by modulating the constant clock frequency. After studying and balancing three modulation profiles of frequency modulation technique, a circuit based on triangular profile in time is proposed. Test results showed that the peak amplitude of the conducted EMI of AC/DC switch-mode power supply (SMPS) decreased by about 3.5~5.7dB with the new circuit. It helps to ease the pressure of diminishing the EMI on the system level of SMPS.