In the petroleum industry, water-soluble polymers can be used as oil displacement agents. However, the use of water-soluble polymers is limited because of poor temperature and salt resistance. To improve temperature and salt resistance, a weakly cationic polymer with large side groups (PAM/AMPS/VI) was prepared by copolymerizing acrylamide (AM) with 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and 1-vinylimidazole (VI). The viscosity of PAM/AMPS/VI water solutions can be increased more than 20 mPa·s compared with PAM/AMPS. In addition, the viscosity of the solution increased continuously after aging at 80 °C, showing good temperature and salt stability. The protonated tertiary amine in the imidazole ring electrostatically interacts with the sulfonic group, increasing the viscosity and salt resistance of the polymer. The five-membered ring of imidazole also enhances the rigidity of the polymer chain and improves the temperature tolerance. As a confirm of the result, a complete spatial network of PAM/AMPS/VI was observed in scanning electron microscopy (SEM) micrographs. Using weak cationic polymers with large side groups can provide a reference for the design of new temperature tolerance and salt resistant polymer.