The invention discloses a method for shearing water gap, which comprises the following steps: 1) arranging a rack at one side of an injection molding machine; 2) arranging a shearing device which is moveably arranged on the rack and is used for taking out a molded product from a plastic mould, shearing a feed port of the molded product, and then placing the molded product in a finished product area; and 3) arranging a control system which is connected to the shearing device and is used for controlling the working state of the shearing device. The invention also discloses a device for implementing the method. The method for shearing water gap provided by the invention has a simple process, is high in speed, has high efficiency, and can be used for greatly shortening the production time and saving the production cost. The device provided by the invention has a simple and reasonable structure. By matching a shearing mechanism with a manipulator, the effect of shearing water gap is achieved. The feed port and the molded product can be automatically moved to the corresponding positions. The degree of automation is high, the repeated working is avoided, the labor intensity is greatly lowered and the product quality is efficiently ensured.