The invention discloses a flatness detection device for celluloid plate processing. The device comprises a processing table, and U-shaped frames are symmetrically and fixedly connected to the surfaces of the two ends of the top of the processing table. Through mutual cooperation of a connecting plate, a first L-shaped rod, a first vertical plate, a first rack plate, a first gear, a driving wheel, a driven wheel and a first fixing mechanism, a strip-shaped plate can drive a celluloid plate to rotate through a positioning clamp until the celluloid plate is turned to a vertical state while a detection plate is reset; and meanwhile, through the mutual cooperation effect of the connecting plate, a second L-shaped rod, a second vertical plate, a second rack plate, a second gear, a driving wheel, a driven wheel and a second fixing mechanism, while the detection plate is reset, the strip-shaped plate can drive the celluloid plate to rotate through a positioning clamp until turnover is completed, therefore, automatic overturning can be achieved, manpower input is saved, and production efficiency is improved.