The invention creates and provides a steel plate flatness detection device, which comprises a detection platform and two sets of moving units, where in each set of the moving unit comprises a first lead screw, a slide base and a first motor, and the motor can drive the slide base to move on the first lead screw; a second lead screw is arranged between the two slide bases, the second lead screw is perpendicular to the two first lead screws, one end of the second lead screw is supported by one of the slide bases, the other end is supported by the other slide bases, a sliding block sleeves the second lead screw, one end of the second lead screw is connected with a second motor, and the second motor can drive the sliding block to move on the second lead screw; and the lower part of the sliding block is connected with a detection unit, and the detection unit comprises a detection rod, a detection head, a movable plate and a detection sensor. The steel plate flatness detection device has the beneficial effects of being simple in structure and convenient to use, ensuring the detection efficiency and detection precision of a steel plate, greatly reducing the labor intensity of the operator, shortening the detection time and being favorable for production.