Preservation solution for free DNA in blood sample

The invention provides a free DNA (deoxyribonucleic acid) preserving fluid in a blood sample, which comprises the following components by weight: 3-10g of anticoagulant, which is one of EDTA-2K, EDTA-2Na and sodium citrate; the nuclease inhibitor is one or more of diethyl pyrophosphate, guanidine isothiocyanate, ammonium sulfate and urea; 0.2 to 0.6 g of purine which is one or two of adenine and guanine; the preservative is one of diazolidinyl urea, imidazolidinyl urea, methylparaben and 2-bromine-2-nitro-1, 3-propylene glycol, and the preservative is one of diazolidinyl urea, imidazolidinyl urea, methylparaben and 2-bromine-2-nitro-1, 3-propylene glycol; 2-4 g of a nutrient substance which is glucose; 0.8 to 1.5 g of a protective agent which is one or more of glycine, trehalose, rhamnose and arginine; the metabolic inhibitor is one or more of glyceraldehyde, sodium fluoride, glyceraldehyde-3-phosphoric acid and phosphoric acid; the buffer solution is supplemented to 100 ml. The plasma free DNA can be preserv