The invention provides a method for preparing a DNA library and specifically capturing and a kit, and the method is suitable for constructing the DNA library of cfDNA, white cell gDNA and DNA from a tissue sample as well as hybridizing and capturing a special target gene area. In the preparation method for the DNA library, terminal repair and 3' end A addition are performed in the same reaction system, and hybridization input is improved, and a blocking preparation in hybridizing and capturing is optimized, so that higher capturing efficiency, higher library variety and more uniform coverage degree are obtained, and therefore, the preparation method is especially suitable for constructing a trace sample library with low-abundance mutation, and is beneficial for realizing saving in technical process. The method provided by the invention can be used for conveniently realizing all variations such as mutation, deletion, insertion, fusion as well as detection on copy number amplification of a target gene, and overcomes the defects that target gene sequences are enriched through a PCR method on DNA level at present, mutation and deletion of the target gene only can be detected, and fusion cannot be detected.