One of the developing trend of microfluidic technology is the integration and miniaturization of the optical detection system. In the meantime, the polarimetric detection of microfluidic system is also one of the important research topics. In this work, the polarized characteristics of polarimetric reflection cavity embedded with Faraday magneto-optic medium is analyzed in detail. It is predicted that such a rotatory reflection cavity has the effect of megneto-optic rotation enhancement. Base on this phenomenon, the method for the angle detection of small amount sample and the theory for the feature analysis of Faraday rotation devices are proposed. The results show that the method can significantly improve the detection sensitivity of magneto-optic rotation media under conditions where the optical path is sieriously restricted. The theoretical limitation of the relative sensitivity with polarimetric detection is 78.5. The method can be applied to the megneto-optical rotation detection of microfluidic system and miniaturize the megneto-optic polarimetric instruments.