Liang, Z.K. Chen, W. Liu, K.F. Liu, X. Qin, B. Zha, J. JACoW Geneva, Switzerland 978-3-95450-167-0 10.18429/JACoW-Cyclotrons2016-MOD01 English 30-32 MOD01 emittance proton simulation cyclotron scattering Contribution to a conference proceedings 2017 2017-01 http://dx.doi.org/10.18429/JACoW-Cyclotrons2016-MOD01 http://jacow.org/cyclotrons2016/papers/mod01.pdf Huazhong University of Science and Technology (HUST) has planned to build a proton therapy facility based on an isochronous superconducting cyclotron. The 250 MeV/500 nA proton beam is extracted from a super-conducting cyclotron. To modulate beam energy, an en-ergy selection system is essential in the beam-line. The simulation based on Geant4 has been performed for the energy selection system and its result will be discussed in this paper. This paper introduces the variation rules of the beam parameters including the beam energy, beam emit-tance, energy spread and transmission. The degrader's gap and the twiss parameter are proven to be effective ways to reduce the emittance after degrader.