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Abstract: In this study, hydrothermal carbon nanospheres (HCNs) were prepared by hydrothermal carbonization using glucose as the precursor, and introduced to improve the properties of water-based drilling fluid for the first time. The variation in rheological and filtration characteristics of water-based drilling fluid with varying concentrations of HCNs were compared between the cases before and after thermal aging. The results demonstrated that HCNs had little influence on the rheological properties of bentonite base mud, but could effectively reduce its filtration loss after thermal aging at 220 °C. For polymer-based drilling fluid, HCNs also exhibited minor influence on the rheology. The H-B model was the best fitting model for the rheological curves before thermal aging. After hot rolling at 220 °C, the viscosity retention rate increased from 29% to 63%–90% with addition of HCNs, and the filtration loss decreased by 78% with 1.0 w/v% HCNs. Meanwhile, the polymer-based drilling fluid with 0.5 w/v% HCNs maintained relatively stable rheology and low filtration loss after statically thermal aging at 200 °C for 96 h. For a bentonite-free water-based drilling fluid prepared mainly with modified natural polymers, the viscosity retention increased from 21% to 74% after hot rolling at 150 °C with 0.5 w/v% HCNs, and was further improved when HCNs and potassium formate were used in combination. The mechanism study revealed that, HCNs could trap dissolved oxygen, scavenge the free radicals and cross link with polymers, which prevented thermal oxidative degradation of polymers and improved the thermal stability of water-based drilling fluid. Meanwhile, HCNs could inhibit clay hydration and swelling in synergy with partially hydrolyzed polyacrylamide by physically sealing the micropores, contributing to shale formation stability. Furthermore, HCNs could effectively improve the lubrication and anti-wear performance of drilling fluid. This study indicated that HCNs could act as green, sustainable, and versatile additives in water-based drilling fluid.