The invention discloses an automatic mud door for a dredger. The automatic mud door comprises a box type mud door, a first pull rod, a second pull rod and a hydraulic oil cylinder; the box type mud door is arranged at the bottom of a mud hopper and is connected with a ship structure through hinges; the bottom end of the hydraulic oil cylinder is connected with the top end of the first pull rod; the bottom end of the first pull rod is connected with the top end of the second pull rod; the bottom end of the second pull rod is connected with the box type mud door; the hydraulic oil cylinder is arranged on a deck cross beam of a ship body; the hydraulic oil cylinder, the first pull rod and the second pull rod are externally provided with circular pipes; the box type mud door comprises a panel,a plurality of reinforcing plates, the hinges and a pull lug; the reinforcing plates are arranged on the panel; the pair of hinges is symmetrically arranged on one side the panel; the panel is connected with the ship structure through the hinges; a sealing rubber strip is arranged at the connecting part of each hinge and the ship structure; and the pull lug is arranged on the upper surface of thepanel. According to the automatic mud door for the dredger, the box type mud door is designed to be opened vertically, and when the automatic mud door is opened, silt of the mud hopper can be quicklydumped, so that the efficiency is high.