The utility model discloses a four-bar mechanism for a water injection propulsion unit astern mechanism, and belongs to the field of water injection propulsion. The mechanism comprises an astern hopper upper jaw, an astern hopper lower jaw, a rudder and a connecting rod. An astern hydraulic cylinder carries out motion of the four-bar mechanism. The upper end of the astern hopper upper jaw is connected with a cylinder rod end of the astern hydraulic cylinder in a hinged mode, the lower end of the astern hopper upper jaw is connected with the upper end of the astern hydraulic cylinder, the connecting rod is connected with the rudder and the lower end of the astern hopper lower jaw, and the astern hopper upper jaw is connected with the rudder. The astern hopper upper jaw, the astern hopper lower jaw, the rudder and the connecting rod form the movable four-bar mechanism. Through the adoption of the movable four-bar mechanism formed by the astern hopper upper jaw, the astern hopper lower jaw, the rudder and the connecting rod, the astern mechanism is small in size and light in structural weight, and the astern efficiency of the astern mechanism is improved.