The conventional acrylamide-based polymers are greatly restricted under the reservoir conditions of high temperature and high salt due to their linear molecular structures. In this article, a grafted water-soluble copolymer based on hydroxypropyl methyl cellulose (HPMC) with excellent thermal viscosifying and salt-tolerant properties at low concentration was prepared via the “graft from” method, with acrylamide (AM) and methyl allyl polyoxyethylene ether (HPEG) as grafted monomers. The structure of the copolymer was identified by 1H nuclear magnetic resonance spectroscopy (1H NMR), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The basic displacement properties of the copolymers under extreme conditions were systematically studied. The results showed that due to the symbiotic effects of the rigid heterocyclic structure of HPMC and the hydrophobic inter-molecular aggregation of thermosensitive HPEG units in the copolymer chain, the copolymer exhibited better thermal thickening ability (the viscosity enhancements changes from 7.1 to 19.9 times from 25 °C to 95 °C in brine solution with total salinity 9350.08 mg.L−1), salt resistance (the viscosity enhancements changes from 1.13 to 2.29 times with salts concentration from 0.0 wt% to 2.0 wt% at 85 °C), shear tolerance (the viscosity retention rate was from 18.6% to 24.2% at 1000s−1) and anti-aging properties (the viscosity retention rate up to 73.7% at 65 °C for three weeks) than polyacrylamide at low concentration (0.2 wt%). Furthermore, the polymer flooding test showed that the copolymer yielded a higher recovery rate of 23.2% higher than that yielded by polyacrylamide (14.5%), elucidating that the copolymer synthesized herein was suitable for enhanced oil recovery in reservoirs with severe conditions.