The invention belongs to the technical field of oil field wastewater treatment, and specifically relates to a degreasing agent of polymer flooding effluent and a preparation method thereof. The degreasing agent is prepared by reacting n-capric aldehyde with epoxy bromobutane under the catalysis of a boron trifluoride diethyl etherate solution to generate an intermediate, and then reacting with pyrrolidone to generate the cationic type degreasing agent. The molar ratio of n-capric aldehyde, epoxy bromobutane to pyrrolidone is 1.0: (0.5 to 1.25): (0.5 to 1.5), preferably 1.0: 1.0: 1.1. The degreasing agent disclosed by the invention has the characteristics of simple preparation technology, low cost, high adaptability and good degreasing effect, and the degreasing reaches up to more than 97%. Therefore, the degreasing agent can be widely applied to a degreasing technology of oil field polymer flooding effluent.