In response to the application requirements of broadband width, low insertion loss and low power consumption of switches proposed by satellite communication, electronic countermeasures and microwave test systems, a wide band RF MEMS switch for the K~D band was designed. The bandwidth of the switch was improved and the loss of the switch was reduced by optimizing the substrate material and the cross-shaped upper electrode structure. The HFSS electromagnetic wave simulation software was used to optimize the geometric parameters of the switch. The results showed that the designed RF MEMS switch could work in the frequency band of 18~188 GHz, the insertion loss was less than 147 dB, the isolation was more than 2012 dB, and its overall volume was about 075 mm3. This switch could be integrated with phase shifters, delay lines, resonators and other devices to realize broadband and low-loss RF reconfigurable MEMS devices and systems, which were used in the fields of next-generation communications and microwave testing.