The proton beam energy determines the range of particles and thus where the dose is deposited. According to the depth of tumors, an energy degrader is needed to modulate the proton beam energy in proton therapy facilities based on cyclotrons, because the energy of beam extracted from the cyclotron is fixed. The energy loss was simulated for the graphite degrader used in the beamline at the superconducting cyclotron of 200MeV in Hefei (SC200). After adjusting the mean excitation energy of the graphite used in the degrader to 76eV, we observed an accurate match between the simulations and measurements. We also simulated the energy spread of the degraded beam and the transmission of the degrader using theoretical formulae. The results agree well with the Monte Carlo simulation.