Bentonite/water-based drilling fluids (BT-WDFs) suffer serious fluid loss particularly at high temperatures, pressure levels, and divalent cation concentrations. The ionic liquid 1-vinyl-3-ethylimidazolium bromide was introduced to synthesize a filtration control additive (PASV) through free radical copolymerization with acrylamide and 2-acrylamido-2-methylpropanesulfonic acid. The effect of the concentration of the ionic liquid on the filtration property of the fluids was determined at high Ca2+ concentrations and temperatures. BT-WDFs added with 3.0 wt% copolymer exhibited excellent performance for controlling bivalent Ca2+-resistant fluid loss and tolerated the influence of Ca2+ up to 70 wt%, which approaches the saturation at room temperature (25 degrees C). As the PASV concentration increased to more than 1.0 wt%, the fluid loss volume was less than 10 mL at room temperature, thereby ensuring safe drilling operation. When the formations drilled were at high temperatures, the filtration control performance was obtained by adding PASV additives. The requisite concentration of PASV varied from 1.5 to 3.0% at 80-150 degrees C. An extremely dense filter cake was produced because of the elimination of the influence of Ca2+ on bentonite and the stabilization of the bentonite dispersion state by the PASV copolymer. These effects improved the filtration control performance of BT-WDFs. Moreover, the fluids exhibited shear thinning behavior over the shear rate range of 0.1-1000 s(-1), resulting in improved practical drilling process. This study provides a basis for applying ionic liquids in the design of additives for drilling fluids.