An adaptive radar phase-comparing method fixed beam micro-nano microwave detecting and demodulating monolithic system of the invention is mainly composed of four parts including a six-port fixed beamcoupler, a microwave frequency detection module, a microwave phase detection module and a microwave power and demodulation module. The adaptive radar consists of an antenna, a transmitting-receiving conversion circuit, a MEMS microwave detection and demodulation monolithic integrated system, a signal memory, a signal analyzer, a microwave signal reconstructor, a microwave signal modulator, and a microwave signal power amplifier. An indirect heating microwave power sensor uses polycrystalline silicon nanowire clusters to form a thermocouple arm, has a thermal conductivity lower than that of a conventional bulk material, has high thermoelectric conversion efficiency, can greatly improve the sensitivity of the microwave detection system, can simultaneously and synchronously detects the power,the frequency and the phase of the microwave signal and demodulates a modulated signal, and achieves the adaptive radar phase-comparing method fixed beam micro-nano microwave detecting and demodulating monolithic system.