The invention provides a method for water-cooling a steel plate over 80mm thick by a quenching machine, belonging to the technical field of thermal treatment of plates with moderate thickness. The method overcomes the problems that the temperature uniformity and stability of the plate cooled are poor, and particularly the plate shape is hard to control. The steel plate over 80mm thick is water-cooled by the quenching machine, and the water yield and the roller speed of each area are set according to the thickness of the steel plate and the temperature of the cooling water, so that the cooling uniformity and the plate shape yield are improved. The method has the advantages that the method is used for varieties such as normalizing boat decks, low alloys and container plate which are 80-150mm thick, so that the plate shape of the water-cooled steel plate is good, and the plate shape yield is greater than 90%, thereby greatly reducing the difficulty of subsequent treatment and lowering the treatment cost of steps such as straightening and pressing steps.