The invention provides a steel plate bending lower die capable of automatically adjusting opening degree, relates to a steel plate bending die, and solves the technical problems that existing lower die opening degree manual adjustment is complex, and the labor intensity is high. Lower surfaces of two lower dies provided by the invention are machined into sawtooth, and upper surfaces of die holders are machined into sawtooth matched with the lower dies; lift cylinder sets and driving mechanisms are mounted between the two lower dies and the die holders; lift connection blocks are tightly fixed with the lower dies, the oil cylinder sets are inserted into the die holders, and T-shaped heads of the oil cylinder sets are matched with grooves of the lift connection blocks; the two lower ides are respectively connected with the two driving connection blocks by bolts, motors are connected with the die holders, gear shafts of the motors are simultaneously engaged with two racks horizontally arranged on the die holders, upper planes of the racks are provided with protruding blocks matched with the grooves of the two driving connection blocks. The steel plate bending lower die capable of automatically adjusting opening degree provided by the invention is used for bending steel plates, and has the advantages that the adjustment is convenient, quick and reliable, the labor intensity of workers is low, and the adjustable opening degree specifications are various.