Background: The FRESCO phase 3 trial demonstrated a significant survival benefit with fruquintinib vs. placebo in the third-line or later therapy of mCRC patients. CEA levels are widely used in conjunction with imaging to monitor response to systemic therapy in patients with mCRC. Herein, we undertook post-hoc analyses of FRESCO patient data to investigate the early changes in CEA during treatment, as well as potential relationships with efficacy parameters. Methods: Patients were included if baseline CEA was abnormal according to local lab reference range. Serum CEA levels were measured at baseline and Day 1 of each cycle (except for Cycle 1). Early CEA change was analyzed at first radiological evaluation (C3D1, Week 8), CEA response was defined as ≥ 50% decrease from baseline, and CEA progression was defined as ≥ 100% increase from baseline. Overall survival (OS) and progression-free survival (PFS) were evaluated by Kaplan-Meier method; hazard ratio (HR) was estimated through Cox proportional hazards model; p-value was generated from log rank test. Results: 88.4% (245/277) and 94.9% (130/137) of patients had an abnormal baseline CEA in the fruquintinib group and placebo group, respectively. Median baseline CEA values were similar between treatment groups. After 2 cycles of treatment, the proportion of patients had CEA response was significantly higher in the fruquintinib group than placebo group (30.0% vs. 1.3%, p < 0.001). In the fruquintinib group, patients with early CEA response (n = 63) had longer median OS (12.8 vs. 7.8 months, HR = 0.45, p < 0.001) and median PFS (5.6 vs. 3.7 months, HR = 0.49, p < 0.001) than patients without (n = 147). 66.7% (140/210) of patients in fruquintinib group had stable disease (SD), and fruquintinib in those patients with concomitant CEA response exhibited a significantly greater OS benefit than with CEA progression (14.4 vs. 8.7 months, HR = 0.38, p = 0.004). Conclusions: Fruquintinib increased early CEA response. CEA response at first radiological evaluation after cycle 2 could be considered as a predictor for better OS and PFS. Among patients with SD at first evaluation, those with CEA response seems benefit more from fruquintinib. Clinical trial information: NCT02314819.