A sealing structure for a plastic packing box, comprising: a packing box body, made by bending a plastic plate; top and bottom openings; and cover plates, configured on sides of each opening and bended inward to stack together to seat the opening, wherein a premade mating frame is respectively engaged with sealing end faces of the outer cover plates configured on two opposite sides of the main body and bended to each other to form a sealing edge, the mating frames are a male-female pair stacked together to sealing the box, ensuring that two end faces is neat after box body sealing, the opposite inner edges of the two mating frames are respectively configured with male and female buckling elements allowed to be pressed together to engage with each other after the stacking, thereby achieving stable and safe buckling, while the cover plate are stacked together simply.