The invention discloses a kit for
bisulfite conversion of
free DNA. The kit comprises a
bisulfite solution, a protective solution, washing liquor A, a
magnetic bead suspension, washing liquor B, washing liquor C and eluate, wherein the protective solution is prepared from the following ingredients: 80% to 85% of
tetrahydrofuran furfuryl alcohol, 10% to 15% of D-
alpha tocopherol cetomacrogol 1000 succinate, 0.2% to 0.3% of tetrahydroglycosyl acetate, 0.05% to 0.1% of ethoxychrysoidine and the balance of water. The invention further provides a method for
bisulfite conversion and
DNA purificationof the
free DNA by using the kit. According to the kit, a
sulfite conversion method is subjected to great technological improvement, so that ctDNA conversion can be completed rapidly (40 to 60 minutes) under mild conditions (70 DEG C to 80 DEG C), pH of a
system can be stabilized, ctDNA degradation is prevented, high-conversion-ratio, high-quality and high-purity
free DNA can be rapidly obtained,and full automated operation of
methylation researches is facilitated.