The theoretical model of the composite microcantilever used in the IR radiation is given based on the theoretical analyses, the infrared absorption characteristics of carbon nanometer tube (CNT) is studied by experiment. A tri-layer microcantilever sensors with CNT film is designed to detect the infrared radiation. The detector is based on the characteristics that the resonance frequency of composite microcantilever shift with the temperature. So, the resonance frequency of microcantilever can be used as sensitive parameter to measure the infrared thermal radiation. The theoretical analysis and simulation results show that this new model can attain power sensitivity of fWtwo orders of magnitude better than the sensitivity of traditional statical ones. Furthermore, the micro-electro-mechanical system (MEMS) sensor could be compatible with integrated circuit technology.