The invention relates to an underwater sand material mixing and conveying device for a dredger and belongs to the technical field of dredging engineering. The device comprises a cavity I, a cavity II,a cavity III, a pipeline I and a pipeline II. A group of control panel I and control panel II arranged in pairs are arranged on the inner side of a top sand material inlet II of the cavity I, and thecontrol panels I and II rotate forward and backward to open and close from the middle folding position of the control panels I and II; the control panels I and II are of arc-shaped thin-walled structures, the arc-shaped openings are downward, pairs of lugs are separately arranged at the axial two ends of the control panels I and II close to the middle folding position of the two control panels, and the two lugs at the same end are connected to corresponding driving devices through a lock chain or a steel wire rope; through holes through which the two lugs penetrate are formed in the top of the cavity I; and guide slots for limiting directional movement of the corresponding lock chains or steel wire ropes are formed in the outer side wall of the cavity II. The device can adjust the mixingeffect of the sand material and reduce the sand material plugging phenomenon, so that a precise hydraulic reclamation operation of the dredger is achieved.