Abstract: The China Institute of Atomic Energy is currently developing a 230 MeV superconducting cyclotron for proton therapy. The RF cavity of 230 MeV superconducting cyclotron is composed of four cavities with helical structure. The RF system applies second harmonic for beam acceleration and the cavity frequency is about 71.25 MHz. The four cavities work in Push-Pull mode, whereas the two opposite cavities are connected directly in the center plane and the other two cavities are connected by bridge under the center area. There is capacitive coupling between the two groups of cavities, and the phase difference of them is 180°. The numerical simulation was used in the design of the four cavities. The accelerating voltage distribution is 75 kV in the center region and 110 kV at the outer radius after optimization, and the unloaded quality factor of the cavity is about 8 800. The oxygen-free copper was selected in the cavity manufacture in order to ensure the RF performance. Cavity welding is the most difficult procedure in the manufacture. The upper (the lower) shell needs to be welded together with a tight tolerance. The mechanical process of the cavities was finished. The unloaded quality factor test of the single cavity shows that the result is better than 7 000, which meets the requirement.