Abstract: A 100 MeV compact proton cyclotron CYCIAE-100 is under design and construction at China Institute of Atomic Energy. The electromagnetic force of the machine was numerically investigated via virtual work method and Maxwell stress tensor method. In practice, the magnetic field distribution was achieved from a FEM code and we got an estimation of the electromagnetic force through virtual work method. Then a new code was written based on Maxwell stress tensor method and implemented in MATLAB environment. The code was used to calculate the electromagnetic force numerically for detailed study. It shows that the results of two different methods are comparable. The attractive force between two opposite poles is larger than the attractive force between a pole and top/bottom yoke, and the difference of the two forces is borne by the bolts which fix the pole on the yoke. This study provides foundation for the calculation of magnet deformation and selection of mechanical structure.