A composite filter-loss reducer, P(ST-g-ACA)/BT, was prepared with corn starch, acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, diallyl dimethyl ammonium chloride and bentonite by monomer intercalation. The filter-loss reducer was characterized with FTIR, XRD , TGA and TEM, the rheological properties and filtration loss of the filter-loss reducer were evaluated. In addition, the mechanism of fluid loss was presented. The results indicated that the filter-loss reducer possessed a “egg coil ” structure and a good thermal stability. Evaluation of P(ST-g-ACA)/BT in a water base drilling fluid showed that, at 2% (mass fraction) concentration of P(ST-g-ACA)/BT, the filter loss of the fresh water-based drilling fluid is 4 mL(normal temperature and medium pressure). The HTHP filter loss of the 10% salt water-based drilling fluid is 22 mL (150 ℃、3.5 MPa). Evaluation results showed that the filter-loss reducer could improve the properties of the thermal stability, salt tolerance, and fluid loss control of water-based drilling fluids. In the 10% salt water-based drilling fluid, the P(ST-g-ACA)/BT is better than other filter-loss reducers in filtration control.