In order to solve the problem that the conducted interference voltage of a high voltage/low voltage (HV/LV) DC-DC converter for vehicles exceeds the standard CISPR25-2016, two different design methods of EMI filter for HV power supply were proposed. The first method is to design a wide-band EMI filter at the high voltage input port of the DC-DC converter. It can achieve the insertion loss of 60dB within 150 kHz-108MHz. Another proposed method is to design a PCB-level EMI filter based on the resonance peaks suppression. During the PCB-level EMI filter design process, a high frequency equivalent circuit model of HV/LV DC-DC converter of EV considering the parasitic parameters was established, and by establishing the transfer functions at key frequencies of 200kHz  and 2MHz the dominated parameters responsible for the over standard points were determined. From simulation and experiment results, the filters designed by above two methods can effectively reduce the conducted disturbance and comply with the limits requirements in 150 kHz-108MHz. What more, the PCB-level filter designed by the second model is smaller in size, only 1/5 of the filter size is designed by the first methods, lower in cost ,and easy to be engineering.