The marine shale of Wufeng Formation and Longmaxi Formation in Sichuan Basin has a good brittleness. After fracturing, the shale can generate complex fracture network and become effective seepage channels for shale gas production. Taking the marine shale of Sichuan Basin as an example, the composition and spectrum of shale minerals were analyzed by scanning electron microscopy, electron probe and cathodic emission. At the same time, the diagenetic evolution of shale was analyzed, and the geological process of brittle formation in the marine shale was found, and the mechanism of autogenous embrittlement of shale was put forward. It is considered that the shale rich in organic matter will undergo autogenous embrittlement in the process of shale evolution to medium-grade and high-grade diagenesis. The transformation process of montmorillonite to illite will produce authigenic quartz, thus improving the brittleness of shale and forming a concealed reticular structure, which provides a basis for later artificial fracturing.