A discharge screening device for a ball mill relates to a screening device for milled material. The discharge screening device comprises a drum screen which rotates together with a mill drum, wherein a discharge end of the drum screen is connected with a reject discharging screen; the reject discharging screen comprises a cylindrical screen body, of which one end is open and the other end is closed; a discharge opening is formed in a baffle at the closed end; a bell mouth is arranged outside the cylindrical screen body; a reject bin is arranged in the cylindrical screen body; the discharge opening is located at one end of the reject bin; a feed opening is formed in each of two sides of the reject bin; material discharged from the drum screen into the reject discharging screen falls into the reject bin via the two feed openings in two sides of the reject bin in the rotation process of the reject discharging screen respectively; two inclined plates are arranged in the reject bin; and the two inclined plates are used for guiding the material, which falls into the reject bin via the two feed openings, to flow into the discharge opening at the other end of the reject bin to be discharged via the bell mouth. The discharge screening device can improve the accuracy of material separation of a drum screening device and effectively finish separation of oversize products and undersize products.