The invention discloses a method for calculating the width of arch dam cracks and determining the thickness of an anti-seepage material. The method comprises the steps that horizontal seams are formed in the positions, where the cracks probably occur, of the surface of an arch dam, original transverse seams of the arch dam and the formed horizontal seams form crisscross seams, and contact point pairs are arranged on the transverse seams and the horizontal seams; a dam body is divided into a plurality of blocks through the multiple transverse seams and the multiple horizontal seams, and the blocks completely segmented out need to be provided with rigid constraints; and solving is conducted through a mixed finite element method, and the rigid body displacement on block bodies and the width of the cracks in the upstream dam surface of the arch dam are obtained. In the past, the transverse seam effect cannot be considered at the same time while the crack width is calculated, usually, the thickness of the anti-seepage material is determined by referring to the thickness, namely, the empirical value, adopted by other engineering, and no theoretical basis is provided. According to the method for calculating the width of the arch dam cracks and determining the thickness of the anti-seepage material, the effect of the transverse seams and the effect of the horizontal seams on the dam body can be considered at the same time, theoretical guidance is provided for determining of the thickness of the anti-seepage material in engineering, and actual benefits can be brought to engineering safety and engineering economy.