[en] Background Considering the nonlinearity of magnet material, the homogeneity of the material and the accuracy of the finite element numerical simulation, sufficient safety margin of the sector is left in the design of the main magnet. In order to obtain ideal isochronous magnetic field after the machining and assembling the magnet, the shimming procedure is necessary according to the actual measured magnetic field. It can ensure the safe implementation of the project. [Purpose] This study aims at the magnet shimming method that is very important for the design and manufacture of the main magnet of cyclotron. [Methods] First of all, the influence of different cutting parameters of magnet pole on the magnetic field in the middle plane was investigated by using the finite element analysis software TOSCA. Then the superposition effect of adjacent grooves on magnetic field was considered to give a numerical method for calculating the shimming value of the sectors. [Result and Conclusion] The feasibility of magnetic field shimming method was verified by simulation, which provides an effective basis for actual magnetic field shimming in the future. (authors)