The invention relates to a bag filling, clamping and expanding mechanism with an automatic bag loading function. The left side and the right side of V-shaped discharging fixing plates of the mechanism are respectively provided with an openable discharging duckbilled plate, the upper ends of the two V-shaped discharging fixing plates are respectively provided with discharging air cylinders, in addition, the lower ends of piston rods are respectively hinged with the middle of a discharging connection pair, the discharging connection pair is in horizontal arrangement, in addition, two ends of the discharging connection pair are symmetrically hinged with discharging connecting rods, the lower ends of the discharging connecting rods are respectively and symmetrically hinged with the middle of the side surface of each discharging duckbilled plate, the outer side of the lower end opening of each discharging duckbilled plate is respectively provided with a bag clamping seat, the outer wall ofthe upper end of each discharging duckbilled plate is respectively and fixedly provided with a bag clamping air cylinder, in addition, the lower end of a piston rod of each bag clamping air cylinder is respectively hinged with the upper end of an L-shaped bag clamping connecting rod, the middle of each L-shaped bag clamping connecting rod is respectively hinged onto the outer wall of each discharging duckbilled plate, the lower part of each L-shaped bag clamping connecting rod is respectively and fixedly connected with the upper end of a bag clamping rubber block installing plate, and the lower end of each bag clamping rubber block installing plate is respectively and fixedly provided with a bag clamping rubber block with sawtooth surfaces. The mechanism has the advantages that bag openings can be conveniently, fast and fixedly arranged on discharging openings, and the opening and the closing of the discharging openings are convenient.