The invention provides a wafer thinning method and device. The wafer thinning method includes the steps that a plurality of chips are machined on one side of a wafer, and a to-be-thinned first semiconductor wafer is obtained; and a third semiconductor wafer of a preset thickness is cut off from a semiconductor substrate of the first semiconductor wafer through a laser source, and a thinned secondsemiconductor wafer is obtained. The laser source is used for forming a burned surface in the semiconductor substrate by a certain depth, the semiconductor substrate is stripped into an upper part anda lower part at the position of the burned surface, and thus the semiconductor substrate is thinned, and electric resistance and heat resistance of the semiconductor chips are lowered; and the situation that a grinding disc is used for grinding and thinning the semiconductor substrate is avoided, the wafer of the preset thickness is cut off directly from the semiconductor substrate, cutting efficiency is improved, and the cut-off wafer can be repeatedly used. The cut-off wafer can be used as a substrate wafer again to manufacture semiconductor chips, and also can be used as seed crystal to grow so as to obtain a crystal ingot, and the crystal ingot can be cut into a plurality of wafers for manufacturing chips, so that the production cost is lowered, and productivity is increased.