Abstract: Using humic acid as raw material, the modified humic acid oil based on fluid loss agent was synthesized under 40 ℃ for 4 h by two step of acyl chlorination and amidation, and the structure of modified humic acid was characterized by infrared spectroscopy. The indoor test shows that the modified humic acid in oil base drilling fluid has good fluid loss effect. When the dosage is 3%, filtration loss is 2.0 mL at medium temperature and medium pressure, fluid loss is 8.0 mL at high temperature and high pressure. The fluid loss agent could resist the high temperature of 160 ℃, and remain low amount of fluid loss under the condition of 20% CaCl₂ or 10% NaCl salt.