The exploration and development of oil and gas resources is continuously deepening, the number of deep and ultra-deep wells is increasing, and the temperature at the bottom of the well is rising. However, emulsifiers, the key treating agents of oil-based drilling fluid (OBDF), degrade and fail at high temperatures, easily leading to the deterioration of drilling fluid performance. In this study, an ultra-high temperature emulsifier (UHT-EM) was synthesized based on substitution reactions. The molecular structure and thermal stability of the UHT-EM were characterized by Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). The stability of the UHT-EM in OBDF was evaluated by interfacial tension, electrical stability (ES) and emulsification rate (ER). The results showed that the interfacial tension of the 3% UHT-EM was 0.3 mN/m. The 0.3% UHT-EM had low viscosity and high shear effect after hot rolling in OBDF at 230 °C, and the ES was 735 V and the ER was 96.3%. This study provides valuable reference and guidance for OBDF to be drilled smoothly in downhole high-temperature environments.