In order to constrain the overvoltage existed in the power supply for the high magnetic field and avoid the equipment in the power supply being destroyed by the overvoltage, this paper presents a method that connecting a RC branch in parallel to the magnet and the way to calculating the parameters of the RC branch. The experimental results show a good agreement with the analysis conclusions, so the method proposed in this paper can well address the overvoltage problem and ensure the safe operation of the power supply.