The utility model relates to the technical field of cranes, in particular to a supporting leg structure of a mobile crane. The device comprises a fixed supporting leg, a fixed plate, a hydraulic cylinder, a movable supporting leg and a folding base plate, the folding base plates have two states of being located on the two sides of the fixed supporting legs and located below the supporting plate. When the folding base plate needs to be used; the folding base plate is unfolded and located below the supporting plate. When the folding base plate is not used, the folding base plate is folded and located on the two sides of the fixed supporting leg, base plate clamping grooves, supporting plates and a plurality of rotary buckles are arranged on the two sides of the fixed supporting leg, the supporting plates are fixedly connected to the lower portions of the two sides of the fixed supporting leg, the base plate clamping grooves are formed in the upper portions of the two sides of the fixed supporting leg, and every four rotary buckles form a group. According to the utility model, the storage of the folding base plate is realized, the occupied space of the folding base plate is reduced, the folding base plate is convenient to carry, the problem that the telescopic range of the movable supporting leg is insufficient is solved, and the mobile crane can better adapt to various working environments.