The invention discloses a coating water repellent agent and a preparation method thereof. The coating water repellent agent is a polyamide polyurea epichlorohydrin resin water solution with the solid content (mass content) of 50 percent to 60 percent, wherein, the mass percentage of organic chloride is less than 0.1 percent, the pH is equal to 6 to 8, viscosity is 150mP question mark s to 450mP question mark s. 10 percent of acid is added as a reaction additive in the reaction, which causes the product weight solid content generated at the final stage of the reaction and the coating property to be improved. Furthermore, low temperature reaction is adopted during the cross-linked reaction process with epichlorohydrin, then slight heating is carried out and ethanolamine is added for furtherreaction, thus reducing the organic chloride content of the generated byproduct, and reducing the organic chloride content of the whole product, which is kept at the level of less than 0.1 percent.