Objective: To analyze the changes in the serum levels of CRH and 5-HT in women with postpartum depression (PPD) and to study the value of the CRH/5-HT ratio for the prediction of PPD. Methods: This prospective study recruited pregnant women from the Fourth Affiliated Hospital, China Medical University between January 2017 and October 2019. Women were considered for inclusion if they had no history, or no current evidence, of a psychiatric disorder. All women were assessed at postpartum day 10 with the Edinburgh Postnatal Depression Scale (EPDS). Blood samples were obtained at 20 weeks of pregnancy and the levels of CRH and 5-HT were determined by radioimmunoassay and ELISA. Associations between EPDS score, the demographic variables, and hormone levels were identified using bivariate logistic regression models. Results: A total of 185 women were included. We found that the serum level of both CRH and 5-HT was significantly correlated with EPDS score; the AUC for CRH was 0.79, and 5-HT was 0.85, which indicated that both CRH and 5-HT are a reliable biomarker for PPD. The AUC, specificity, and sensitivity of CRH/5-HT were 0.92, 0.86, and 0.95, respectively, which were better than those of CRH or 5-HT individually. Conclusions: We believe that the serum CRH/5-HT ratio is an excellent biomarker for the prediction of PPD.