An on-line MEMS microwave power detection system with wide dynamic range was designed. The equivalent model of MEMS cantilever beam was established. The overload power of MEMS cantilever structure was studied theoretically, and the relationship between overload power and beam size was obtained. Based on this relationship, MEMS beams of different sizes were designed to improve the upper limit of system power detection, so as to improve the dynamic range of on-line microwave power detection system. According to the structural parameters of the system, the dynamic range of the system was 0~8.41 W. The simulation model of microwave power detection system was established. The S-parameter simulation results show that the return loss of the system is from -37.61 dB to -46.15 dB and the insertion loss is from -0.28 dB to -0.16 dB at 8~12 GHz. It has a wide dynamic range and good microwave performance. This work has a certain reference value for the research of microwave power detection system based on MEMS beams.