Abstract: Not only the levels of cirgulating DNA in patients suffer from tumor is higher than healthy, but circulating DNA has changes in gene structure.The characteristic changes of circulating DNA in oncology have clinical significance in early diagnosis of tumor, curative effect evaluation and follow-up of cancer.The article is reviewed about the source and releasing mechanism of circulating DNA, genetically modified variation, level change, circulating DNA integrity, the research Methods and limitation of circulating DNA.