Background: ctDNA liquid biopsy technology has been widely used in the whole course of cancer management, from early screening and early diagnosis of tumors to tumor monitoring and medication guidance. However, the performances of ctDNA as a biomarker in these scenarios are different among different cancer types. One possible reason is that different types of cancer release different amounts of ctDNA or cfDNA, therefore, we explored the cfDNA levels in different cancer types in this study. Methods: We collected 15913 peripheral blood samples from 14686 cancer patients with different clinicopathologies, 256 patients with pulmonary nodules, and 971 healthy people. We isolated their plasma and extracted the cfDNA using QIAamp Circulating Nucleic Acid Kit, Apostle MiniMax cfDNA Extraction Kit or HaploX magnetic bead method cfDNA extraction kit. We calculated the average concentration of cfDNA (ng/ml) for different groups. Results: The number of samples of cervical cancer, liver cancer, glioma, colon cancer, prostate cancer, nasopharyngeal carcinoma, gallbladder carcinoma, melanoma, cholangiocarcinoma, colorectal cancer, breast cancer, gastric cancer, endometrial carcinoma, lung cancer, urothelial carcinoma, rectal cancer, ovarian cancer, healthy people, and pulmonary nodule patients are 93, 469, 113, 277, 139, 76, 89, 132, 171, 4010, 407, 547, 68, 7750, 46, 203, 96, 971 and 256, respectively. The cfDNA level of corresponding groups are 88.07, 79.51, 75.57, 63.58, 61.37, 59.09, 57.59, 57.16, 42.67, 34.73, 31.75, 31.37, 31.33, 29.84, 28.72, 28.69, 23.83, 17.82 and 12.54 ng/ml, respectively. The main groups are shown in the table below. Conclusions: The cfDNA concentrations of healthy people or pulmonary nodule patients were significantly lower than that of tumor patients, and there were also significant differences between different types of cancer. What's interesting, the cfDNA concentrations of lung cancer patients, pulmonary nodule patients and healthy people were quite different. What’s more, there were differences between Colon cancer and Rectal cancer patients. So, the cfDNA concentration maybe a good dimention to be used in early cancer screening and tracing of tumor types.