Abstract: To further improve temperature resistance, dispersion ability and filtration reduction of fluid loss additive in water-in-oil emulsion drilling fluids, fluid loss additive H-QA was prepared by the reaction of humic acid with octadecyl trimethyl ammonium chloride, to use in low toxicity water-in-oil emulsion drilling fluid. The influence of synthetic conditions on the fluid loss additive properties was explored, and the properties of fluid loss additive in low toxicity invert oil-emulsion drilling fluid were evaluated according to the petroleum and national gas industry standards. The results of optimal synthetic conditions were showed as follows: the ratio of potassium humate to octadecyl trimethyl ammonium chloride （1 831） is 3﹕1, pH of the reaction system 8.86, reaction temperature 40℃, scission reaction time 4h. When addition of H-QA is 4%, the API filtration reduced from 6.4 mL to 4.0 mL, the HTHP filtration reduced from 8.6 mL to 6.8 mL. It shows a good fluid loss control effect in water-in-oil emulsion drilling fluid. No influence on drilling fluid rheology was noticed by addition of the fluid loss additive, which has certain application and research value.