The invention discloses a water-proofing agent for coated paper coatings and a preparation method thereof. The preparation method comprises the following steps of: (1) reacting diethylenetriamine with organic acid in the presence of a catalyst at a reaction temperature of between 120 and 130 DEG C for 30 to 60 minutes, and then collecting an unsaturated polyamide intermediate in a reaction product; and (2) adding urea at the temperature of between 80 and 100 DEG C to perform a heat-insulating reaction at the temperature of between 100 and 110 DEG C for 8 to 10 hours, performing temperature reduction until the temperature reaches between 30 and 40 DEG C, adding epoxy chloropropane to perform a reaction at the temperature of between 60 and 70 DEG C for 30 to 60 minutes, regulating the pH value by using acid until the pH value is between 6 and 8 to prepare the water-proofing agent for the coated paper coatings, and adding water into the water-proofing agent to regulate solid content of solution to 55 to 60 percent. After the water-proofing agent is used for production, paper and paperboards which are disassembled from a machine are not needed to be cured, products do not contain formaldehyde, and no adverse effect is executed on coating operation, product storage and application environment.