The invention relates to a polyacrylamide oil displacement agent and a preparation method thereof. The polyacrylamide oil displacement agent provided by the invention has a polyacrylamide structure with the co-existence of a crosslinked network structure formed with a multifunctional crosslinking agent as the crosslinking center and a branched chain structure. Specifically, the crosslinked structure accounts for 60%-95% in the whole polyacrylamide structure, and the multifunctional crosslinking agent is obtained by reaction of allyl glycidyl ether or (methyl) glycidyl acrylate and polyethylenepolyamine. The oil displacement agent provided by the invention has high elasticity, viscosity and deformation restorability at the same time, also can ensure excellent long-term aging resistance ina high temperature and high salinity environment, and is conducive to application in tertiary oil recovery.