Abstract: The present invention provides a sequencing library, and the sequencing library has an inserted fragment which is an equidirectional alternating concatemer of a sequence to be tested and a tag sequence. The present invention further provides a method for preparing the sequencing library. The present invention also provides a sequencing method. The sequencing library and sequencing method as provided in the present invention are capable of removing DNA amplification errors and sequencing errors under any sequencing depths, so that mutations of DNA molecules could be ultra-accurately determined. The sequencing library of the present invention is suitable for construction of a sequencing library of trace short DNA fragments and even of single-strand DNAs.