The invention discloses a polyamine inhibitor for water-based drilling fluid and a method for preparing the polyamine inhibitor. A structural formula of the polyamine inhibitor is shown, wherein an n can be 0 or 1 or 2 or 3; an m ranges from 100 to 200; an R<1> is -CH<3> or -CH<2>CH<3> or -CH<2>CH<2>OH. The method for preparing the polyamine inhibitor for the water-based drilling fluid includes mixing polyamine organic compounds and water with one another to obtain mixtures; slowly adding epichlorohydrin into the mixtures; enabling the epichlorohydrin and the mixtures to react with one another; adding chain growth terminators into the mixtures; cooling products until the temperatures of the products reach the room temperature after reaction is completed so as to obtain the polyamine inhibitor for the water-based drilling fluid. The polyamine inhibitor for the water-based drilling fluid and the method have the advantages that the polyamine inhibitor is excellent in shale inhibition and can be normally used under the condition of temperature of 180 DEG C; the method is simple and is easy to implement.