The object of this work was to improve the temperature and salt resistance of filtrate reducers used in solid‐free water‐based drilling fluids at 200°C. This copolymer (PAD) was obtained by reacting pentaerythritol triallyl ether (PTE) with acrylamide AM, N‐isopropyl acrylamide (N‐AM), methacrylic acid (MAA), and 2‐acrylamide‐2‐methylpropanesulfonic acid sodium (AMPS‐Na) in radical polymerization with potassium peroxydisulfate (KPS) and sodium bisulfite as initiators. The molar ratio of each monomer shown as follow: nAm:nN−Am:nMAA:nAmps: = 1:1.6:0.9:1.5. The functional groups of each monomer were revealed by Fourier transformation infrared. Moreover, 13C nuclear magnetic resonance confirmed the PAD was synthesized by addition polymerization through these monomers. Scanning electron microscopy showed that the surface of the filter cake was smooth and dense, indicating that PAD can effectively block small pores and thus reduce filter loss. The analysis suggested the PAD produced a “ball” structure at high temperature and salt which controlled the filtration of drilling fluid. The maximum temperature resistance boundary of PAD was 210°C. PAD maintained filtration performance in nearly saturated sodium chloride, 1.4 g/cm3 HCOOK and 1.37 g/cm3 HCOONa, at 210°C for 16 h.