This study aimed to assess the prognostic value of the postoperative serum carcinoembryonic antigen (CEA) levels/preoperative serum CEA levels ratio (CEA ratio) in colorectal cancer (CRC) patients with high preoperative serum CEA levels and to identify the optimal prognostic cutoff value. The medical records of 187 CRC patients in a single center who underwent surgery between September 2012 and September 2014 were retrospectively reviewed. CEA ratio was defined as the ratio between the postoperative serum CEA and preoperative serum CEA. The optimal cutoff values for the CEA ratio were determined by time-dependent receiver operating characteristic (ROC) curve analyses. The Chi-square test or Fisher's exact probability test were used to test the correlation between CEA ratio and clinicopathological characteristics. Univariate, multivariate, and subgroup Cox proportional hazards analysis were used to identify independent prognostic factors. Kaplan-Meier method was used for establishing survival curves. The median follow-up time was 62 months (range 3-88 months). The optimal CEA ratio cutoff value closely related to disease-free survival was 0.295. In the Chi-square test, the CEA ratio was associated with pN stage (p=0.003) and postoperative CEA (p<0.001). In the multivariate analysis, the CEA ratio was an independent prognostic factor for disease-free survival (p=0.003, HR 2.300 [95% CI: 1.326-3.988]) and cancer-special survival (p=0.003, HR 2.525 [95% CI: 1.381-4.614]). The CEA ratio reflected the prognosis of CRC patients more accurately than postoperative CEA levels alone, and the CEA ratio of 0.295 was more likely to reflect the prognosis than other cutoff values. The CEA ratio is a simple and useful tool for further forecasting the prognosis of CRC patients with high preoperative CEA levels and may help develop strategies for the postoperative treatment of CRC patients.