The RF characteristic of a RF MEMS switch with a package cap was studied in this paper based on simulation and analysis. Simulation was divided into two parts-the edge of the cap and the center of the cap, in order to observe the different effect level between the edge and the center of the cap on the switch. The thickness of the bonding layer and the depth of cavity were changed in simulations so as to detect the key factor which affects the effect level. We combined the simulation results with the microwave theory, calculated the changes of the switch impedance caused by the package cap. The simulated and calculated results were validated using the soft MicroWave Office.