The invention relates to a method for quantitatively determining the methylation rate of a nucleic acid through sequencing. According to the invention, the method comprises at least the following steps: a) treating the nucleic acid with a chemical reagent or an enzyme containing solution, whereby the base pairing behavior of methylated cytosine bases and/or unmethylated cytosine bases of the nucleic acid are altered such that methylated cytosine bases become distinguishable from unmethylated cytosine bases, and b) introducing into the nucleic acid at least one base for generating a sequencing signal to be used as a reference signal for normalization, and c) sequencing the nucleic acid, whereby a signal from each cytosine base of the nucleic acid, or a signal from each guanine base of the nucleic acid and a reference signal from the at least on introduced base is obtained, and d) normalizing the signal obtained from each cytosine base of the nucleic acid, or the signal obtained from each guanine base of the nucleic acid to the reference signal from the at least one introduced base.