The invention relates to a kit for assay circulating DNA (deoxyribonucleic acid) in maternal peripheral blood and a library creation method. The library creation method and the kit for assay circulating DNA in maternal peripheral blood can be used for assaying circulating DNA in maternal peripheral blood, for example prenatal diagnosis. The method for creating a DNA library for sequencing includes the following steps: single-3' end A addition is carried out on at least part of a blunt-end DNA fragment, so that a DNA fragment with A added to a single 3' end is obtained; and blunt-end ligation is carried out on the DNA fragment with A added to the single 3' end obtained in Step B-1.