Traditional polymer materials struggle to meet the increasingly complex demands of downhole environments，while inorganic nanomaterials exhibit poor dispersion. Grafting nanomaterials onto polymer chains or embedding polymers into nanomaterials to form polymer nanocomposites can leverage the synergistic effects of both materials to enhance the overall performance of water-based drilling fluids. This paper introduced the application of polymer nanocomposites on improving the rheology，optimizing the plugging and filtration reduction，and enhancing the collapse resistance of water-based drilling fluids，and then summarized their effects and mechanisms. The mechanisms by which polymer nanocomposites improved drilling fluid rheology included polymers enhancing the dispersion of solid nanoparticles in drilling fluid，increasing the internal friction between fluid layers，and forming a spatial network structure through hydrogen bonding and electrostatic interactions between polymers and clay particles. The mechanisms for optimizing the plugging and filtration reduction of drilling fluids involved using inorganic nanomaterials for micropore plugging to form a dense mud cake，while the strong adsorption and amphiphilic properties of polymers created a hydrophobic membrane on the mud cake surface. The mechanisms for enhancing the collapse resistance of drilling fluids included three aspects. First，a dense barrier was formed through nanoparticles bridging and blocking shale nanopores which prevented water molecules from invading the shale. Second，the multifunctional groups on polymer chains formed competitive adsorption on clay surfaces to reduce clay adsorption of water molecules. Third，the combined effects of mature polyamine polymer amino embedding and cationic double electric layer compression strengthened shale hydration inhibition. Challenges of polymer nanocomposites included potential uneven dispersion of inorganic nanoparticles in the polymer matrix during large-scale production. Finally，the paper forecasted the research directions of polymer nanocomposites in three areas，such as reducing wear and drag，reservoir protection，and sustainability.