(Objective) To address the challenge of reusing fracking flowback water, where the high salinity causes a sudden drop in the viscosity of friction reducers and a decline in their performance, and to achieve the efficient recycling of fracking flowback water. (Methods) A temperature- and salt-resistant drag-reducing agent was synthesised using a dispersion polymerisation method, with acrylamide (AM) forming the main chain of the copolymer and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) optimising hydrophilicity and salt resistance. A third hydrophobic monomer was introduced, and the optimal synthesis scheme was determined through comparative screening of the copolymer systems’ properties; Regarding performance testing, a six-speed viscometer was used to determine the apparent viscosity of the copolymer, whilst a rheometer was employed to assess its high-temperature rheological properties and evaluate its thermal stability. Scanning electron microscopy (SEM) was utilised to examine the microstructure of the copolymer and analyse its spatial structural characteristics in high-salt environments. Concurrently, field applications were conducted to verify its practical performance. (Results and Conclusions) Screening experiments identified octadecyl dimethyl allyl ammonium chloride (DMAAC-18) as the key functional monomer; by leveraging the synergistic effects of hydrophobic association and electrostatic interactions, it significantly improves the salt tolerance and rheological properties of the drag-reducing agent. The optimal synthesis conditions were a monomer mass ratio of AM:AMPS:DMAAC-18 = 9:1:0.15, a monomer concentration of 25 wt%, and a polymerisation temperature of 35 °C. Under these conditions, the P18 terpolymer prepared exhibited an apparent viscosity of up to 183.6 mPa·s, with the high-temperature rheological final viscosity stabilising at 36 mPa·s. Microscopic characterisation results confirm that this terpolymer can stably form a continuous porous network structure in high-salt media, effectively enhancing the fracking fluid’s salt resistance and sand-carrying capacity. Field application at Well H1 demonstrated that the slickwater fracturing fluid formulated with this temperature- and salt-resistant friction reducer exhibited stable and reliable performance during operation. The fracturing operation resulted in significant increases in oil and gas production, providing a viable technical solution and theoretical support for the resource recovery and reuse of shale oil fracturing flowback fluids.