In the drilling industry, the demand for environmentally friendly additives with high thermal stability is increasing due to the dual factors of increasing environmental pressure and high-temperature oil layers. However, commonly used non-toxic and biodegradable additives, such as etherified modified starch, cannot withstand temperatures higher than 150 °C. Additionally, natural polymers with better thermal stability obtained through graft modification with sulfonated monomers face challenges in meeting the standards of toxicity and biodegradability. To address these technical problems, a novel graft and crosslink copolymer, St-AA/AM/NVP/MBA (SAANM), was synthesized from corn starch by combining graft modification with a non-sulfonated monomer and cross-linking modification. Laboratory evaluation results confirm that the thermal stability of SAANM in a nitrogen atmosphere was close to 300 ℃, and it exhibits excellent temperature resistance up to 170 °C in bentonite-based mud, while also retaining the non-toxic and biodegradable characteristics of starch. The water-based drilling fluid (WBDF), added with SAANM, demonstrated outstanding rheological properties, fluid loss control performance, and environmental friendliness after aging at 170 °C and being polluted by high concentrations of NaCl or CaCl2. The successful application of SAANM in a high-temperature directional well in an offshore oil field confirms its potential for borehole cleaning and wellbore stability.