In order to meet the demand for high-temperature environmentally friendly drilling fluids in deep oil and gas development, this study utilized natural black fungus as a raw material and employed liquid–solid extraction and enzymatic hydrolysis processes to prepare a natural polysaccharide material, designated as AABG. The comprehensive performance of AABG in drilling fluids was systematically evaluated through indoor experiments including rheology, lubricity, and environmental properties. The results of indoor experiments indicate that AABG meets the requirements of both environmental protection and high-temperature performance, maintaining good filtration control and lubricity at 180 °C, with low biotoxicity and good biodegradability, thereby meeting environmental standards. By utilizing scanning electron microscopy and infrared spectroscopy, the anti-high-temperature mechanism of AABG was preliminarily studied: AABG addition enhances particle dispersion in drilling fluids, inhibits clay particle aggregation, and improves filter cake quality. Fungal polysaccharides exhibit better high-temperature resistance compared to other common polysaccharides, attributed to the unique molecular structure and strong intra- and inter-molecular hydrogen bonds of AABG. Under high-temperature conditions, the molecular chains of fungal polysaccharides undergo a transition from ordered to disordered conformation, exposing more functional groups to form hydrogen bonds with water molecules, thereby improving their high-temperature stability. This study preliminarily demonstrates that the natural fungal polysaccharide extract AABG exhibits better high-temperature resistance compared to traditional polysaccharide treatment agents, and explores its anti-high-temperature mechanism, providing a new technical route for the development of high-temperature environmentally friendly drilling fluid treatment agents with certain innovation and application prospects.