The copolymer (PAAN-SiO2) of acrylamide (AM), 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS), N-vinylpyrrolidone (NVP) and modified nano-silica (M-SiO2) was synthesized by free radical polymerization in a water solution. The composition, micro-morphology and thermal stability properties of PAAN-SiO2 were characterized by Fourier transform infrared spectroscopy, thermal gravity analysis and transmission electron microscopy (TEM). The results showed that AM, AMPS and NVP were successfully grafted onto the surface of M-SiO2 and formed a spherical core-shell structure copolymer. A significant reduction in the filtration volume was achieved after PAAN-SiO2 added, and this phenomenon was even more pronounced after aging at the high temperature. The filtration volume of base slurry containing 2wt% of calcium chloride after aging at 180 degrees C was reduced from 186 to 6ml after adding 2wt% of PAAN-SiO2, which exhibited a special property of anti-calcium contamination at high temperature. The interaction mechanism between PAAN-SiO2 and bentonite and calcium ions was analyzed by particle size analysis, scanning electron microscopy, TEM and X-ray diffraction. Because of the stretching of polymer chain under 180 degrees C that more amide, sulfonic acid groups and cyclic rigid groups were exposed and the strengthening of the connection between PAAN-SiO2 and clay prevents a large amount of ion exchange between Ca2+ and clay layers, reducing the agglomeration of clay. Meantime, the distribution of clay particles was more extensive and some particles with a size of 1-10m plugged the micro-nano-pores on the filter cake, and eventually the thin and compact filter cake was formed.