Background: Cancer-Related Fatigue (CRF) is a prevalent and debilitating symptom in breast cancer patients, significantly impacting their quality of life and potentially affecting postoperative outcomes. Despite advances in breast cancer treatment, the long-term implications of CRF on prognosis remain unclear. Methods: This retrospective cohort study included 310 postoperative breast cancer patients from two large comprehensive hospitals. We systematically collected demographic, medical history, lifestyle, psychological status, and CRF data. Univariate and multivariate Cox regression analyses were performed to identify independent prognostic factors. Random forest survival analysis, propensity score matching, Kaplan-Meier survival analysis, and restricted cubic spline analysis were utilized to comprehensively evaluate the relationship between CRF and postoperative prognosis. Results: Higher CRF scores were associated with worse postoperative outcomes (HR = 1.07, 95% CI: 1.03-1.10, P < 0.001). Other independent prognostic factors included higher HADS scores (HR = 1.09, 95% CI: 1.01-1.17, P = 0.032), not undergoing breast-conserving surgery (HR = 0.43, 95% CI: 0.20-0.89, P = 0.024), and not receiving endocrine therapy (HR = 7.13, 95% CI: 2.64-19.22, P < 0.001). Random forest survival analysis highlighted CRF as the most critical variable impacting prognosis. Propensity score matching effectively balanced baseline characteristics, and Kaplan-Meier survival curves demonstrated significantly lower survival rates in high-CRF patients both before and after matching (P < 0.05). Restricted cubic spline analysis revealed a nonlinear relationship between CRF scores and disease progression risk, with significant risk increases beyond specific thresholds. Conclusions: CRF is a crucial predictor of disease progression in postoperative breast cancer patients. Higher CRF and HADS scores, non-breast-conserving surgery, and lack of endocrine therapy are associated with poorer prognosis. These findings underscore the importance of comprehensive postoperative care, including effective management of fatigue and psychological health, to improve patient outcomes. Future research should focus on prospective studies and targeted interventions for CRF to validate these findings and enhance clinical practices.