Aiming at the problems of few types of anti-ultra-high temperature oil-based drilling fluids (OBDFs) for deep ultra-deep drilling, immature supporting technology and insufficient long-term stability at ultra-high temperatures, an anti-ultra-high temperature and long-term stable OBDFs was developed. The synthesis methods and mechanisms of the key additives were described, and the performance evaluation was carried out. The modified fatty acid primary emulsifier and fatty acid amide auxiliary emulsifier were prepared by using unsaturated fatty acid, maleic anhydride, and fatty alkylamine. They exhibited good emulsion stability. The demulsification voltage was higher than 400V. Amphiphilic rheology modifier was prepared by the reaction of dimer fatty acid with diethanolamine, and aliphatic diamine, which could improve the aggregation state of emulsion droplets, with low viscosity and high gelling effect. The water-based acrylic resin was modified by ester monomers to prepare a high-temperature resistant composite fluid loss agent, which was stable in oil and had outstanding fluid loss performance. Furthermore, a set of OBDFs with temperature resistance of 240°C was constructed with these developed treatment agents as the core materials. The viscosity and gel strength of this drilling fluid were stable. The HTHP fluid loss was less than 5 mL, and the sedimentation stability was good after vertical aging at 240°C for 5 days. The demulsification voltage was higher than 1000 V. This research can provide guidance for the development of safe and efficient drilling and completion fluids for deep wells.