The invention relates to the technical field of a bottom supporting structure with supporting legs, and particularly relates to a heightening device for a crane. The device comprises a back plate andside plates, wherein a bottom plate is in sliding connection between the two side plates; a sliding chute, a swing rod, a push rod, slide rods, a baffle, spacing shafts, a motor and a rotating rod driven by the motor are arranged on the side plates; slide blocks are fixedly connected on two sides of the bottom plate, are used for extending into the sliding chute and are in sliding connection withthe sliding chute; a swing groove is formed in the slide block; a fixed shaft is fixedly connected to the wall of the sliding chute; one end of the swing rod is in rotating connection with the fixed shaft, and the other end of the swing rod is hinged with one end of the push rod; the swing rod is hinged with the baffle; slide rods are arranged on two sides of the bottom of the baffle; the motor isarranged above the sliding chute; the rotating rod is fixedly connected with the motor; the other end of the rotating rod is hinged with the swing rod; spacing shafts are arranged on two sides of theside plates, and the slide rods pass through the spacing shafts and are in sliding connection with the spacing shafts. According to the scheme, the balance of a crane can be kept in the process of heightening the crane.