Abstract: A 325 MHz Spoke cavity at medium and low β is developed in institute of High Energy Physics (IHEP) for the CiADS linac. Through the study on the principle of designing a superconducting cavity, three kinds of Spoke cavities including single Spoke cavity, double Spoke cavity and 3-bar Spoke cavity, were designed by the CST MWS.Although both the 3-bar Spoke and the double Spoke have a 3-gap structure, their Spoke bars are different. In the process of cavity design, parametric modeling was used to construct the cavity shape. Then the cavity shape was optimized by the parametric sweep in the CST MWS to minimize ratio of peak electric field to accelerating gradient (E_P/E_acc) while keeping ratio of peak magnetic field to accelerating gradient (B_P/E_acc) at a reasonably low range. Through the comparison of RF performance parameters, a double Spoke structure was selected as the design, and further optimization for electromagnetic field of double Spoke cavity is carried out.