In this paper, the design, optimization and equivalent circuit model for a K-band capacitive RF MEMS switch are presented. The fixed-fixed switch is designed with folded structures on the high-resistance silicon substrate, by using gold as the beam and silicon nitride as the dielectric layer. The resulting switch working on 25.2GHz exhibits the performance with insertion loss less than 0.20dB, isolation more than 40dB and the drive voltage less than 16V by simulation.