Zhang, L.G. Chen, W. Fan, K. Hu, S. Li, L.X.F. Mei, Z.Y. Qin, B. Zeng, Z.J. JACoW Geneva, Switzerland 978-3-95450-182-3 10.18429/JACoW-IPAC2017-WEPVA081 English 3446-3448 WEPVA081 Contribution to a conference proceedings 2017 2017-05 http://dx.doi.org/10.18429/JACoW-IPAC2017-WEPVA081 http://jacow.org/ipac2017/papers/wepva081.pdf Main magnet is the heaviest component in a superconducting cyclotron, which occupies a large amount of cost. Topology optimization method is implanted to minimize the weight of main magnet while keep the field performance, which will make significant economic benefit. Due to the powerful superconducting coils, the main magnet is driven into saturation, and the nonlinear effect of the material must be considered. If the ordinary standard density method is used for the main magnet structure optimization, the nonlinear B-H relation have to be interpolated and the sensitivity analysis is very complicated. In this paper, a proper 2D model is established and the optimization formulation is given using standard density method. Then, the optimized topology of the main magnet for a 250MeV superconducting proton cyclotron is designed.