Abstract: In order to meet the requirement of proton beam energy for single event effect testing, a 300 MeV/A H⁺₂ superconducting cyclotron was designed at China Institute of Atomic Energy. The cyclotron applies superconducting coil to reduce the size of main magnet and provides variable energy proton beams by stripping extraction H⁺₂ ions. Beam extraction dynamics was studied by adjusting the stripping point and analyzing the proton trajectory and beam envelop, and the physical design of extraction was accomplished. The results show that the cyclotron is able to extract variable energy protons in 205-240 MeV and 265-300 MeV continuously, and retain the ability of mono-energy extraction at lower energy range.