Since the single-pole multi-throw switch (SPMT) needs to meet the performance requirements of broadband and high isolation when it is used to switch filters and transmission lines in phased array radars and bandwidth transceivers, a broadband and high isolation MEMS single-pole three throw switch (SP3T) was designed. The MEMS SP3T switch was optimized by using the HFSS module in ANSYS electromagnetic simulation software, and the mechanical properties of the upper electrode were simulated by COMSOL software. The simulation results show that the designed MEMS SP3T switch can work in the frequency band of 1-90 GHz. The insertion loss is less than 1 dB@90 GHz, the isolation is greater than 35 dB@90 GHz, and the overall volume is about 075 mm3.