Abstract: The design of a asymmetric RF cavity requires a code that can treat 3D structure . This paper describes the characteristics , functions and applications of the 3D RF cavity computer - aided design ( CAD ) program MAFIA . The 3D geometry design of non - axial symmetric RF cavity with a coupler is realized by CAD method . A kind of data input is presented that can save not only machine memory ,but also the CPU time , and large programs can be run on a medium -small size machine . The numerical simulation of a L - band RF cavity is presented and the modeling results agree well with its experimental values . We have also compared electric properties of the RF cavity with variant geometry by applying 3D numerical and graphic simulations .The influence on electric and magnetic fieldsby adding a input coupler or HOM coupler at the side of a RF cavity is analysed . Adding a coupler to a RF cavity can cause the change of resonatancefrequency . A method that can compensate the variation of frequency is also presented .