The invention discloses a construction method and detection method of a trace fragmented 
DNA methylation detection 
library. The 
library construction method of the present invention comprises: linkingfragmented 
DNA to a 
methylation linker, wherein all cytosines on the 
methylation linker are methylated cytosines, the methylation 
linker includes a 
long chain and a short chain which are complementaryto form a 
double chain, and the 
long chain includes a sequencing platform universal sequence, a single molecule tag sequence, a sample tag sequence and a sequencing primer binding sequence; and the 
DNA linked with the methylation linker is treated with 
sodium bisulphate, so that the 
cytosine C in a non-methylated CpG island is converted into 
uracil U; and a specific primer and a universal primerare used for PCR amplification enrichment of a target region subjected to 
sodium bisulfite treatment and containing a 
CpG site to be detected, so that a 
library available for on-
machine sequencing isobtained. According to the invention, methylation of trace DNA taken through 
liquid biopsy can be detected, and the 
CpG island methylation status of multiple regulatory regions of 
multiple tumor suppressor genes can be quantitatively detected simultaneously.