Sha, P. Liu, B. Mi, Z.H. Zhai, J.Y. Zhang, X.Y. Zheng, H.J. JACoW Geneva, Switzerland 978-3-95450-191-5 10.18429/JACoW-SRF2017-TUPB036 English 463-465 TUPB036 cavity ion cryomodule SRF experiment Contribution to a conference proceedings 2018 2018-01 https://doi.org/10.18429/JACoW-SRF2017-TUPB036 http://jacow.org/srf2017/papers/tupb036.pdf CEPC will use 650 MHz cavities for the collider (Main Ring) and 1.3 GHz cavities for the Booster. Each booster cryomodule contains eight 1.3 GHz 9-cell cavities, which is similar as LCLS-II. Each collider cryomodule contains six 650 MHz 2-cell cavities, which is totally new. So our R&D of CEPC cavity mainly focuses on the 650 MHz 2-cell cavity. A cryomodule which consists of two 650 MHz 2-cell cavities has began in early 2017. In this thesis, the RF and mechanical design is displayed with Helium Vessel. Besides, multipacting is analyzed. In order to achieve high Q, N-doping is also studied.