Colon cancer is one of the most common malignant tumors with an increasing incidence rate in China. Surgical resection is still the main treatment for colon cancer at present. Radical surgery followed by three/six months chemotherapy is the standard of care for stage III colon cancer; however, patients with different risk factors have different prognosis. The IDEA trial divided stage III colon cancer patients into low-risk (T1-3/N1) and high-risk (T4 or N2) groups, and showed that for some low-risk patients, three months chemotherapy did not decrease survival outcomes, while for some high-risk patients, the recurrence risk was still high even after six months chemotherapy. Therefore, it's worth to explore other risk stratification factors beyond T and N stage for these patients. Circulating tumor DNA (ctDNA) is derived from cancer cells and can be detected in blood. Literatures have reported that ctDNA can be used for tumor diagnosis, therapeutic monitoring, and prognosis assessment in multiple cancers, including colon cancer. The GERCOR-PRODIGE trial, an accompanying study of IDEA, reported that in the high-risk group of stage III colon cancer, patients with ctDNA-positive and receiving six months chemotherapy had similar prognosis to these with ctDNA-negative and receiving three months chemotherapy; in the low-risk group, patients with ctDNA-negative and receiving three or six months chemotherapy had similar prognosis to patients with ctDNA-positive and receiving 6 months chemotherapy, but patients with ctDNA-positive and receiving three months chemotherapy had the worst prognosis. The results of this trial suggests that ctDNA can be potentially used as a further stratification factor to guide adjuvant chemotherapy for stage III colon cancer. Several RCTs have shown that double-drug regimens chemotherapy based on oxaliplatin (FOLFOX and XELOX) can improve the prognosis of patients with stage III colon cancer. Therefore, the ESMO, NCCN, and CSCO guidelines recommend FOLFOX or XELOX for stage III colon cancer. The 2-year disease-free survival rate of these patients who received FOLFOX or XELOX chemotherapy was about 80%. It is worth to further explore how to improve the prognosis of these patients. Recently, the triple-drug regimens of oxaliplatin, irinotecan, and fluoropyrimidine (FOLFOXIRI) has been found to be superior to FOLFOX or XELOX for metastatic colorectal cancer in terms of response rate and survival. Currently, research on FOLFOXIRI plus targeted therapy in metastatic colorectal cancer is progressing rapidly, but there is little research on the use of FOLFOXIRI as adjuvant chemotherapy for stage III colon cancer. There is an ongoing international multicenter phase III RCT comparing FOLFOXIRI and FOLFOX6 adjuvant chemotherapy for high-risk stage III colon cancer patients, but it did not further stratify patients based on postoperative ctDNA status, which may result in some patients receiving excessive chemotherapy. The present study plans to enroll patients with stage III colon cancer with positive ctDNA within 1 month after surgery. These patients will receive 2 cycles of XELOX chemotherapy followed by retesting ctDNA. During the waiting period of the ctDNA results (approximately 3 weeks due to the testing time), all patients will receive another cycle of XELOX chemotherapy. If the ctDNA remains positive, the patients will be randomly assigned to receive 8 cycles of FOLFOXIRI as intensified adjuvant chemotherapy or 5 cycles of XELOX regimen as standard adjuvant chemotherapy. If the ctDNA is negative, the patients will continue to receive 5 cycles of XELOX chemotherapy. Within 3 weeks after the completion or termination of chemotherapy, ctDNA will be retested again. The aims of this study are to explore the value of ctDNA in surveillance of chemosensitivity and to preliminarily evaluate whether the intensified chemotherapy with FOLFOXIRI can increase ctDNA clearance as well as its safety in stage III colon cancer.