The invention relates to a wharf lower portion dredging device. The wharf lower portion dredging device comprises diesel oil driven equipment, two slurry pumps, a high-pressure water pump, a hydraulic pump, a conveying mechanism, a hoisting mechanism and a drag head assembly. The diesel oil driven equipment, the slurry pumps, the high-pressure water pump and the hydraulic pump are arranged on a working platform. The two slurry pumps are mounted on the rear side of the conveying mechanism. The front end of the working platform is connected with the conveying mechanism through a movable shaft. The working platform is provided with a front anchor gear, a rear anchor gear, a left anchor gear and a right anchor gear for fastening and fixing. A dredging construction method comprises the steps that firstly, equipment is mounted and debugged; secondly, the dredging device enters the construction area, and front splay anchors / cables and rear splay anchors / cables are fastened; thirdly, the dredging drag head assembly is extended into a preset coordinate point according to the tide level; fourthly, construction is conducted in a segmented and layered mode; and fifthly, after the drag head assembly is fallen, the drag head assembly is translated left and right by fastening or loosening the anchor cables, and the step is repeated to complete operation. According to the scheme, during operation, operators do not participate in operation on the wharf lower portion, and the whole implementation process is easy to operate, safe and reliable.