The invention discloses a design method for the width of a slit of a single-slit dam for preventing and treating a debris flow with driftwood. The single-slit dam comprises a dam body foundation and a dam boy main body, wherein the dam body main body is provided with the slit which is along the flow direction of the debris flow and halves the dam body main body. The design method for the width of the slit comprises the following steps of firstly determining the average length of the driftwood, the average diameter of the driftwood, the volume ratio which is accounted for by a solid matter in a debris flow body, the total amount of the debris flow and the total amount of the driftwood, and afterwards, calculating to obtain the width of the slit of the single-slit dam through a calculation formula for the width of the slit. According to the method, the blockage burst effect of the driftwood on the single-slit dam is sufficiently considered; a relation among the seven of the driftwood on the blockage burst of the slit, the width of the slit, the average length of the driftwood, the average diameter of the driftwood, the total amount of the driftwood, the total amount of the debris flow and the volume ratio which the solid matter accounts in the debris flow body is comprehensively considered; a blockage burst discrimination index is combined on the basis; the width of the slit of the single-slit dam in different conditions of the debris flow with the driftwood can be reasonably determined; further, the design method is simple, convenient and effective.