Inclusion Criteria:
1. Age ≥ 18 years, any gender.
2. Patients with metastatic colorectal adenocarcinoma confirmed by pathological histology or cytology.
3. Expected survival time ≥ 12 weeks.
4. ECOG score of 0-2.
5. Previously treated for metastatic colorectal cancer with fluoropyrimidine (allowing intravenous and/or oral fluoropyrimidine formulations, excluding DPD enzyme inhibitors), irinotecan, and oxaliplatin chemotherapy, which failed (treatment failure defined as intolerable adverse reactions, disease progression during treatment, or disease progression within 6 months after completing adjuvant chemotherapy); regardless of prior use of targeted drugs such as cetuximab or bevacizumab.
6. Patients must have an interval of at least 2 weeks since the last chemotherapy (at least 1 week for oral chemotherapy drugs) or more than 4 weeks since the end of radiotherapy, with the study's observable lesions located outside the radiotherapy target area.
7. According to RECIST 1.1 criteria, at least one measurable tumor lesion with a maximum diameter ≥ 1 cm as determined by spiral CT scan.
8. Laboratory test results within 1 week before enrollment must meet the following criteria:
1. Hemoglobin ≥ 90 g/L; Platelets (PLT) ≥ 75 × 10\^9/L;
2. White blood cells (WBC) ≥ 3.0 × 10\^9/L; Neutrophils (ANC) ≥ 1.5 × 10\^9/L;
3. Serum creatinine (Cr) ≤ 1.5 × upper limit of normal (ULN);
4. Total bilirubin (TBI) ≤ 1.5 × ULN; Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 2.5 × ULN (≤ 5 × ULN if there is liver metastasis).
9. No prior use of raltitrexed or S-1 (or DPD enzyme inhibitors) in the treatment of colorectal cancer.
10. Signed informed consent.
Exclusion Criteria:
1. Patients unable to take oral medications.
2. Patients who have previously been treated with small molecule TKI drugs.
3. Patients with severe hepatic or renal insufficiency, or a recent history of myocardial infarction (within 3 months).
4. Patients with a history of other malignancies within the past five years, except for cured cervical carcinoma in situ and basal cell carcinoma of the skin.
5. Patients with a history of inflammatory bowel disease or extensive colonic resection, ≥50% or extensive small bowel resection with chronic diarrhea, or intestinal obstruction.
6. Patients with severe uncontrolled internal medical conditions or acute infections (fever \> 38°C due to infection).
7. Patients with symptomatic brain or leptomeningeal metastases (unless the patient has been treated for brain or leptomeningeal metastases \> 6 months, with negative imaging results within 4 weeks before study entry, and has stable clinical symptoms related to brain or leptomeningeal metastases at study entry).
8. Patients with clinically significant, uncontrolled pleural effusion or ascites despite clinical intervention.
9. Pregnant or breastfeeding women, or patients of reproductive potential (males or females not in menopause for less than 1 year) unwilling to use contraception.
10. Patients known to be allergic to raltitrexed, S-1, and Fruquintinib or any of their components.
11. Patients deemed unsuitable for participation in this clinical trial by the investigator.
