The invention relates to a cutting device for a steel plate of a crane. The cutting device comprises a support and supporting legs perpendicularly connected with the support. Cutting trolleys are arranged on the two sides of the support. A walking motor is arranged on the outer side of one end of each cutting trolley. The upper ends of the cutting trolleys are connected with a beam. A cutting gun head is arranged on the beam. A supporting plate adjusting device is arranged on the inner side face of each cutting trolley. Fixing gears are arranged on the inner sides of the upper ends of the supporting legs. Fixing chains are arranged between the fixing gears. The ends of the fixing chains are fixedly connected with the supporting legs. The inner sides of the fixing chains are provided with L-shaped chain plates. Supporting plates are connected between the L-shaped chain plates. The supporting plate adjusting device comprises adjusting teeth; main shafts are arranged in the centers of the adjusting teeth; rotating motors are connected with the other ends of the main shafts; the ends of the main shafts are connected with supporting shafts A and supporting shafts B through gears; and the ends of the supporting shafts A and the ends of the supporting shafts B are provided with supporting teeth A and supporting teeth B respectively. The cutting device has the beneficial effects of being simple in structure, convenient to use and operate and reasonable in design.