The utility model relates to a flying shear length measuring device of a transverse cutting unit. The flying shear length measuring device comprises a roller which rotates along with a steel plate and an encoder. The flying shear length measuring device of the transverse cutting unit is characterized in that a movable support, a fixed support and an air cylinder are further arranged, the movable support is provided with two supporting rods which are provided with bearing seats and an encoder mounting rod, and the two supporting rods and the encoder mounting rod are fixed on a cross rod of the movable support; the roller which rotates along with the steel plate is a length measuring roller which is provided with a central spindle longer than the length measuring roller, the distance between the two supporting rods is larger than the length of the length measuring roller, the length measuring roller is mounted on the bearing seats of the movable support through the central spindle, the encoder mounting rod is stretched from the end opposite to the encoder mounting rod, and the encoder is mounted on the central shaft and rotates along with the central shaft; the fixed support is in a rectangle shape, the air cylinder is mounted on the fixed support, a piston rod of the air cylinder is fastened with the cross rod of the movable support and is perpendicular to the cross rod. The flying shear length measuring device of the transverse cutting unit is accurate in measuring.