The invention discloses an underwater blasting soil sampling device and method of a soil body under foot blades of an open caisson, and relates to the technical field of bridge foundation construction. The device comprises the open caisson, wherein the open caisson comprises an open caisson wall; the foot blades are arranged at one end of the open caisson wall; the open caisson is provided with a plurality of guide holes; the guide holes penetrate through the top end of the open caisson wall and the foot blades; and bracket supports are arranged at the top end of the open caisson wall. The device further comprises blasting areas; the open caisson is pressed against the blasting areas; the blasting areas are provided with blasting holes located below the foot blades and communicating with the guide holes; and the blasting areas are further provided with blasting grooves adjoining to the inner wall of the open caisson wall. The underwater blasting soil sampling device of the soil body under the foot blades of the open caisson can avoid the diving operation in the blasting process and greatly improve the soil sampling efficiency of the soil body under the underwater foot blades.