Packaging of MEMS has been identified as one of the most significant areas of research for enabling MEMS usage in product applications. In order to make MEMS a real-life opportunity, it is vital to explore and develop an understanding of the possibilities and limitations of MEMS packaging. This paper presents several packaging structures for RF MEMS switch which based on GaAs substrate. The return loss of X-band RF MEMS switch before and after packaged can be simulated by Ansoft HFSS. The results show that return loss of RF MEMS switch after packaging deteriorated at least 3dB compared with that of before. Thermal mismatch caused by the variation of the temperature in the packaging process will introduce additional thermo-elastic strain and geometric deformation into the mechanical structure. The influence on return loss of the RF MEMS switch is researched in this paper too. Considering that, return loss simulated by HFSS decreases from -16.38dB to -16.88dB. Then, some design guidelines of RF MEMS packaging derived from the simulation of HFSS are also concluded at the end of the article.