A kit for determining ctDNA concentration and a method for determining ctDNA concentration is disclosed, wherein the method comprising dissolving extracted ctDNA in Tris-HCl buffer with pH>7.0 and adding into the buffer 2,7-bis(1-methyl-4-vinylpyridine)-9-ethylcarbazole iodised salt as a molecular probe, calculating the concentration of ctDNA in the solution by measuring the Fluorescence intensity of the solution. It has extremely high sensitivity in the determination of 0 μg/ml˜50 μg/ml ctDNA solution.