The invention discloses a large-scale open caisson high-pressure water jetting foundation-clearing device and an open caisson foundation-clearing method thereof, and relates to the technical field of bridge construction. The large-scale open caisson high-pressure water jetting foundation-clearing device comprises a dredging pipe, a water jet pipe and at least one driving device, the upper end of the dredging pipe is rotatably fixed, the bottom end of the dredging pipe is provided with at least two cross beams, and a dredging port is formed in the bottom of the dredging pipe; the driving device is fixed to the cross beams, a movable beam is slidably arranged at the end, away from the dredging pipe, of at least one of the cross beams, and the driving device is connected with the movable beam and drives the movable beam to slide; and the water jet pipe extends to the cross beams or the movable beam along the upper end of the dredging pipe, the water jet pipe is connected with at least one nozzle, the nozzles are fixed to the cross beams or the movable beam, and each nozzle comprises a straight-head nozzle towards the wall of a well and an elbow head nozzle facing a base. According to the large-scale open caisson high-pressure water jetting foundation-clearing device and the open caisson foundation-clearing method thereof, the problem that the amount of equipment is large in site operation is solved, and clearing of the open caisson can be completed at one time.