Abstract: In this paper, the general design consideration of 50 MeV H^- cyclotron (CYCIAE-50) was briefly introduced, and the optimization design process and results of the main magnet were presented. After the main parameters of main magnet of CYCIAE-50 were selected, the model of main magnet was established and the isochronous magnetic field was calculated on central plane. The peak field and local saturation of main magnet were optimized to reduce the construction and operation cost by adjusting parameter of magnet. The magnetic pole angle and gap were also optimized to make H^- meet both the radial and vertical focusing requirements and to avoid crossing harmful resonances. In the end, the structural deformation of the main magnet was calculated and its influence on beam dynamics was also presented. According to the design results, the design of main magnet of CYCIAE-50 can meet the requirements, which can provide an important reference for the design and construction of other subsystems.