Abstract: To get the magnetic field distribution for beam dynamics calculation in the (100 MeV) compact cyclotron featured with a complex geometric magnet, we simplified the magnet structure and used various 3D finite element meshing methods in the initial design stage. After the results comparison by different meshing methods, we analysed numerically the three dimentional magnetic field and found that this 3D finite element calculation process could meet the requirement of the physics design for 100 MeV compact cyclotron.