bstract：The preparation techniques of hydroxypropyl starch(HPS) by half-dry method and solvent method were introduced, and the low fluid loss performance was studied and compared. The result shows that half-dry method was a good method. The optimum reaction conditions were as follows: reaction temperature 55℃, reaction time 8h, the ratio of m(sodium hydroxide) to m(starch) 0.014, the ratio of m(propylene oxide) to m(starch) 0.30, water content in reaction system 31%, The MS of product is 0.33. The product could resist temperature up to 150℃ in 4% NaCl salt water muds, while the product made by solvent method only to 140℃. Compared with solvent method product at 140℃, it has excellent capacities of salts , and could be used in saturated-salt solution, the maximum content of Ca2+ and Mg2+ was 930mg/L and 360mg/L, respectively. Further studies show that the product had a good low fluid loss performance when MS is over 0.2. The fluid loss performance of HPS is related to the MS and the synthesis process.