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As an important slurry material for water-based drilling fluids, montmorillonite has been widely used in oil drilling because of its excellent properties such as expansibility, dispersion and suspension, slurry making, cation exchange and adsorption. However, at a certain temperature and pressure, montmorillonite will transform into illite, chlorite and other minerals, changing the properties of drilling fluids. In order to systematically summarize and sort out the current research status of montmorillonite transformation, this study focused on the research progress of montmorillonite transformation into other minerals under different temperature and pressure conditions, and summarized the transformation mechanism of montmorillonite. The montmorillonite is transformed into illite by adding K+ in the interlayer spacing and removing Si4+ by adding Al3+ to tetrahedron structure. The conversion of montmorillonite to chlorite by adding Fe2+, Mg2+ ions into the octahedron, and Si4+ in the tetrahedron is replaced by Al3+. The precipitated brucite layer is connected with the upper and lower layers.