A method of constructing a sequencing-library, comprising: fragmenting a nucleic acid sample, to obtain a nucleic acid fragment; ligating the nucleic acid fragment to an adaptor, to obtain a nucleic acid fragment ligated to the adaptor, wherein the adaptor is a methylation index adaptor; subjecting the nucleic acid fragment ligated to the adaptor to sequence capturing using a probe, to obtain a nucleic acid fragment from a predetermined region; subjecting the nucleic acid fragment from the predetermined region to a bisulfite treatment, to convert an unmethylated cytosine in the nucleic acid fragment from the predetermined region to a uracil, to obtain a converted nucleic acid fragment; and amplifying the converted nucleic acid fragment, to obtain an amplified product, wherein the amplified product constitutes the sequencing-library, wherein ligating the nucleic acid fragment to the adaptor comprises: end-repairing the nucleic acid fragment, to obtain an end-repaired nucleic acid fragment; adding a base A to the end-repaired nucleic acid fragment at 3'-end, to obtain a nucleic acid fragment having the base A at 3'-end; and ligating the nucleic acid fragment having the base A at 3'-end to a Paired-end index methylated adaptor, to obtain the nucleic acid fragment ligated to the adaptor, wherein the nucleic acid fragment is end-repaired using T4 DNA polymerase, Klenow fragment and T4 polynucleotide kinase, wherein the base A is added to the end-repaired nucleic acid fragment at 3'-end using Klenow fragment (3'-5' exo-) polymerase and dATP, and wherein the nucleic acid fragment having the base A at 3'-end is ligated to the Paired-end index methylated adaptor using T4 DNA ligase. fragmenting a nucleic acid sample, to obtain a nucleic acid fragment; ligating the nucleic acid fragment to an adaptor, to obtain a nucleic acid fragment ligated to the adaptor, wherein the adaptor is a methylation index adaptor; subjecting the nucleic acid fragment ligated to the adaptor to sequence capturing using a probe, to obtain a nucleic acid fragment from a predetermined region; subjecting the nucleic acid fragment from the predetermined region to a bisulfite treatment, to convert an unmethylated cytosine in the nucleic acid fragment from the predetermined region to a uracil, to obtain a converted nucleic acid fragment; and amplifying the converted nucleic acid fragment, to obtain an amplified product, wherein the amplified product constitutes the sequencing-library, wherein ligating the nucleic acid fragment to the adaptor comprises: end-repairing the nucleic acid fragment, to obtain an end-repaired nucleic acid fragment; adding a base A to the end-repaired nucleic acid fragment at 3'-end, to obtain a nucleic acid fragment having the base A at 3'-end; and ligating the nucleic acid fragment having the base A at 3'-end to a Paired-end index methylated adaptor, to obtain the nucleic acid fragment ligated to the adaptor, end-repairing the nucleic acid fragment, to obtain an end-repaired nucleic acid fragment; adding a base A to the end-repaired nucleic acid fragment at 3'-end, to obtain a nucleic acid fragment having the base A at 3'-end; and ligating the nucleic acid fragment having the base A at 3'-end to a Paired-end index methylated adaptor, to obtain the nucleic acid fragment ligated to the adaptor, wherein the nucleic acid fragment is end-repaired using T4 DNA polymerase, Klenow fragment and T4 polynucleotide kinase, wherein the base A is added to the end-repaired nucleic acid fragment at 3'-end using Klenow fragment (3'-5' exo-) polymerase and dATP, and wherein the nucleic acid fragment having the base A at 3'-end is ligated to the Paired-end index methylated adaptor using T4 DNA ligase.