The invention discloses an automatic quantitative dust discharging device for a negative-pressure dust collector. The automatic quantitative dust discharging device comprises a flexible sealing plate, a dust blocking plate, a rigid sealing plate, a crossbeam and a quadrilateral telescopic frame; an air inlet and an air outlet are formed in a dust collector box body, the air inlet is positioned below the air outlet, a dust removal device is installed at the upper part of the dust collector box body, the dust blocking plate is positioned at the lower part of the dust collector box body, and a filter element is installed between the dust removal device and the dust blocking plate; the flexible sealing plate and the rigid sealing plate are respectively fixed at two sides of the dust blocking plate, the top end of the dust blocking plate is fixed on the crossbeam, a hinge shaft at the lower end part of the quadrilateral telescopic frame is connected with the crossbeam, and the hinge shafts at two side end parts of the quadrilateral telescopic frame are respectively fixed on the dust collector box body and a piston; two ends of the crossbeam are installed in a slide way formed in the dust collector box body; and an empty box is arranged at one side of the bottom of the dust collector box body, a piston is installed between the empty box and the dust collector box body, and the piston is connected with a regulating valve. The automatic quantitative dust discharging device has the beneficial effects that the automatic sealing and quantitative dust discharging of the dust collector are realized by virtue of the flexible sealing plate.