Disclosed are a mesh belt plane gyratory screen with the function of removing small impurities first, and a grain impurity removal method thereof. The mesh belt plane gyratory screen comprises a screen frame (1), wherein a screen box (2) is suspended from the screen frame (1) via rocker rods (9), swing rods or steel wire ropes; the screen box (2) is internally provided with a small-impurity mesh belt screen (4) and a large- and medium-impurity mesh belt screen (3); the screen box (2) is connected to a plane gyratory mechanism (5); two sides of the large- and medium-impurity mesh belt screen (3) and/or the small-impurity mesh belt screen (4) are connected to a side plate (22) via a sealing structure (6); the large- and medium-impurity mesh belt screen (3) and the small-impurity mesh belt screen (4) are both connected to a driving mechanism (8), an oblique impurity discharge sliding plate (17) is provided below a screen surface of the small-impurity mesh belt screen (4); and the bottom of the impurity discharge sliding plate (17) is provided with a discharging mechanism.