Objective:To compare the difference between the combined diagnostic effect of plasma Septin9(SEPT9) and polyligand Syndecan-2(SDC2) methylation with four serum tumor markers in the diagnosis of colorectal cancer.Methods:In this study, 128 patients who were treated in the affiliated Hospital of Xuzhou Medical University from March to December in 2019 were selected for a case-control study.All the subjects were examined by gastroenteroscopy.According to the pathological results, they were divided into three groups: colorectal cancer group( n=74) and colorectal adenoma group( n=7). The patients with no abnormal or inflammatory polyps or proliferative polyps examined by gastroenteroscopy were taken as the control group( n=47). The methylation levels of SEPT9 gene and SDC2 gene were detected by Roche Lightcycler 480 II real-time fluorescence quantitative polymerase chain reaction, and the concentrations of alpha-fetoprotein, carcinoembryonic antigen, carbohydrate antigen 125 and carbohydrate antigen 199 were detected by Roche Cobas 8000 electrochemiluminescence instrument.Chi-square test was used to compare the positive rate of each marker in the three groups.Medcalc was used to draw the receiver operating characteristic curve (ROC) curve of the subjects′ working characteristic curve, and the value of each index in the diagnosis of colorectal cancer was analyzed. Results:The positive rates of SEPT9 gene and SDC2 gene methylation were 81.1%(60/74) and 67.6%(50/74) respectively in colorectal cancer group, and increased to 85.1%(63/74) after combined detection.The positive detection rates of alpha-fetoprotein, carcinoembryonic antigen, carbohydrate antigen 125 and carbohydrate antigen 199 in colorectal cancer group were 1.4%(1/74), 33.8%(25/74), 6.8%(5/74) and 13.5%(10/74), respectively.When the four tumor markers were detected together, the positive detection rates were only increased to 43.2%(32/74), except for AFP and carbohydrate antigen 125(χ 2=3.847, 2.430, all P>0.05). The differences were statistically significant (χ 2=48.230, 30.487, 43.285, 3.847, 8.788, 6.988, 8.722, all P<0.05). The area under the curve (AUC) of SEPT9 methylation, SDC2 methylation, alpha fetoprotein, carcinoembryonic antigen, carbohydrate antigen 125 and carbohydrate antigen 199 were 0.854 (0.781, 0.910), 0.795 (0.715, 0.861), 0.575 (0.485, 0.662), 0.685 (0.597, 0.764), 0.603 (0.513, 0.689) and 0.631 (0.541, 0.715), respectively.The AUC of combined detection of two DNA methylation markers was better than that of alpha fetoprotein, carcinoembryonic antigen, carbohydrate antigen 125 and carbohydrate antigen 199, and the differences were statistically significant (alpha fetoprotein: Z=4.990, P<0.001; carcinoembryonic antigen: Z=3.743, P<0.001; carbohydrate antigen 125: Z=4.951, P<0.001; carbohydrate antigen 199: Z=3.983, P<0.001). The combined detection of two kinds of gene methylation was better than the combined detection of four kinds of serum markers in the diagnosis of colorectal cancer, and the difference was statistically significant ( Z=3.334, P<0.001). Conclusion:The combined detection of SEPT9 gene and SDC2 gene methylation in plasma is more suitable for non-invasive diagnosis of colorectal cancer than the combined detection of 4 serum tumor markers.