A method for determining the nucleic acid composition in a total nucleic acid mixture comprising a first nucleic acid and a second nucleic acid, in which said first nucleic acid and said second nucleic acid are derived from different sources, said method comprising: 1) treating said total nucleic acid mixture with a bisulfite, to convert non-methylated cytosine in said total nucleic acid mixture into uracil, and obtain a converted total nucleic acid mixture; 2) subjecting said converted total nucleic acid mixture to multiplexed fluorescent quantitative PCR using a first set of amplification primers and a second set of amplification primers, to capture and amplify a predetermined nucleic acid fragment, and obtain a ratio R of a methylated amplification product to a non-methylated amplification product of said predetermined nucleic acid fragment, wherein said first nucleic acid and said second nucleic acid both contain said predetermined nucleic acid fragment, and the predetermined nucleic acid fragment in said first nucleic acid differs from the predetermined nucleic acid fragment in said second nucleic acid in terms of the methylation level; said first set of amplification primers specifically recognize the converted predetermined nucleic acid fragment, and said second set of amplification primers specifically recognize the unconverted predetermined nucleic acid fragment; and a methylation proportion M 1 of said predetermined nucleic acid fragment in the first nucleic acid and a methylation proportion M 2 of said predetermined nucleic acid fragment in the second nucleic acid are predetermined; and 3) based on the ratio R of the methylated amplification product to the non-methylated amplification product of said predetermined nucleic acid fragment, the methylation proportion M 1 of said predetermined nucleic acid fragment in the first nucleic acid, and the methylation proportion M 2 of said predetermined nucleic acid fragment in the second nucleic acid, determining the nucleic acid composition in the total nucleic acid mixture; wherein in Step 3), the content ε of the first nucleic acid in the total nucleic acid mixture is determined according to the formula ε=(M 2 +RM 2 -R)/[R(M 2 -M 1)-(M 1 -M 2)]. 1) treating said total nucleic acid mixture with a bisulfite, to convert non-methylated cytosine in said total nucleic acid mixture into uracil, and obtain a converted total nucleic acid mixture; 2) subjecting said converted total nucleic acid mixture to multiplexed fluorescent quantitative PCR using a first set of amplification primers and a second set of amplification primers, to capture and amplify a predetermined nucleic acid fragment, and obtain a ratio R of a methylated amplification product to a non-methylated amplification product of said predetermined nucleic acid fragment, wherein said first nucleic acid and said second nucleic acid both contain said predetermined nucleic acid fragment, and the predetermined nucleic acid fragment in said first nucleic acid differs from the predetermined nucleic acid fragment in said second nucleic acid in terms of the methylation level; said first set of amplification primers specifically recognize the converted predetermined nucleic acid fragment, and said second set of amplification primers specifically recognize the unconverted predetermined nucleic acid fragment; and a methylation proportion M 1 of said predetermined nucleic acid fragment in the first nucleic acid and a methylation proportion M 2 of said predetermined nucleic acid fragment in the second nucleic acid are predetermined; and 3) based on the ratio R of the methylated amplification product to the non-methylated amplification product of said predetermined nucleic acid fragment, the methylation proportion M 1 of said predetermined nucleic acid fragment in the first nucleic acid, and the methylation proportion M 2 of said predetermined nucleic acid fragment in the second nucleic acid, determining the nucleic acid composition in the total nucleic acid mixture;