The invention relates to a method for constructing a DNA library used for sequencing, the DNA library and a sequencing method. According to the method for constructing the DNA library, through newly designed tag connectors, the original way of one PCR amplification reaction for each sample is changed into the way of one PCR amplification reaction for a plurality of samples during mixed sequencing of a great number of samples; so the usage amount of reagents used for library construction is reduced, and labor cost for sequencing is lowered.