The approaching era of sustainable development and rapid increase in global energy demand necessitates the development of high-performance drilling fluids with low toxicity, sustainability, and in-situ rheology controllability for complicated formation excavation. Herein, smart water-based drilling fluids (WDFs) with thermocontrollable rheological properties are developed using bentonite (BT) and dual-functionalized cellulose nanocrystals (fCNCs). The fCNCs are synthesized by surface grafting of poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) and poly(N-isopropylacrylamide) (PNIPAM) through free-radical graft polymerization. The presence of PAMPS grafts with abundant amide and negatively charged sulfonate groups not only enables fCNCs to attach on the surface of BT platelets creating BT/fCNC clusters but also ensures the created BT/fCNC clusters to uniformly disperse through electrostatic repulsion. On the other hand, the introduction of thermoresponsive PNIPAM grafts induces the association of BT/fCNC clusters at elevated temperatures through enhanced hydrophobic attraction, resulting in promising thermothickening rheological performance of BT/fCNC-WDFs. Furthermore, the thermothickening rheological behavior can be maintained even after 10 heating/cooling cycles, albeit with a slight increase in the critical transition temperature. The developed BT/fCNC-WDFs with sustainability, in-situ rheology controllability, and good cyclability have great potential in the smart drilling industry, making the exploration and production of oil and gas more safe, efficient, and environmentally friendly.