This study was carried out to investigate cell-free DNA (cfDNA) as a potential biomarker for colorectal cancer diagnosis.Patients with colorectal cancer (n = 25) who had not undergone surgery, and 35 patients with postoperative colorectal cancer were enrolled. Peripheral blood samples were collected from the colorectal cancer subjects (experimental group), and also from 30 healthy volunteers (control group). Quantitative PCR (qPCR) was used to determine cfDNA concentration and integrity in each group. The cfDNA levels of the two groups were analyzed to determine the relationship between the cfDNA and the clinical features of colorectal cancer patients. The receiver operator curve (ROC) was used to analyze sensitivity and specificity of cfDNA, carcinoembryonic antigen (CEA), cancer antigen 199 (CA199) and cancer antigen 125 (CA125). cfDNA concentration and cfDNA integrity in patients with colorectal cancer before surgery were significantly higher than those in patients with colorectal cancer after surgery, and cfDNA concentration of colorectal cancer patients after surgery was also significantly higher than that of the healthy control group, but the integrity was not significantly different from the control group. There was no significant correlation between cfDNA concentration/integrity and gender, age, disease stage, tumor location, tumor differentiation, and expressions of cancer antigen 153 (CA153), neuron specific enolase (NSE) and alpha fetoprotein (AFP) in colorectal cancer patients before or after surgery. However, there was a significant correlation between the expression levels of CEA/CA125 and concentration of cfDNA. The CA199 expression level was significantly correlated with cfDNA integrity. The sensitivity and specificity of cfDNA and integrity were higher than those used for traditional tumor biomarker detection. cfDNA concentration is significantly increased in serum of colorectal cancer patients. Thus, it may serve as a potential indicator of colorectal cancer.