Disclosed are a mesh belt plane gyratory screen with an upper small-impurity screen and a grain impurity removal method thereof. The mesh belt plane gyratory screen with an upper small-impurity screen comprises a screen frame (1), wherein the screen frame (1) is connected to a screen box (2) via hanging rods (9), rocker rods, swing rods, or steel wire ropes; the screen box is internally provided with a small-impurity mesh belt screen (4) and a large- and medium-impurity mesh belt screen (3); the screen box is connected to a plane gyratory mechanism (5); the large- and medium-impurity mesh belt screen and/or the small-impurity mesh belt screen are/is provided with blocking flanges (6); an oblique impurity discharge sliding plate (17) is provided below a screen surface of the small-impurity mesh belt screen; and the bottom of the impurity discharge sliding plate (17) is provided with a discharging mechanism (18). The mesh belt plane gyratory screen is used for removing impurities from grain in the later period.