The present invention discloses a grain weighing device for a grain water content determination device. The grain weighing device comprises a charging barrel, and a feed port and a discharge port arerespectively arranged at an upper end and a lower end of the charging barrel; a blocking device is arranged at the discharge port, and the blocking device is mainly formed by a left turning plate anda right turning plate which are arranged in a bilateral symmetry mode; and a left side of the left turning plate and a right side of the right turning plate are hinged to and connected with the charging barrel through hinges. A cylinder is employed to drive a cylinder shaft to pull up or press down so that closure and opening of the left turning plate and the right turning plate can be achieved and blanking and storing can be achieved, a weight difference obtained by the weighing sensor is namely the weight of materials in the charging barrel in the storing state, and therefore a purpose of effective weighing is achieved, the whole device can achieve effective weighing and can achieve free discharge, the degree of freedom of the grain weighing device is higher than that of a traditional device, and the accuracy and the actual efficiency are better.