Objective To investigate the clinical significance of serum B cell activating factor （BAFF） and complement lytic factor Bb in patients with idiopathic membranous nephropathy （IMN）， as well as their correlation with the urinary albumin?to?creatinine ratio （UACR）. Methods A total of 204 IMN patients were enrolled as the study group， and 100 healthy individuals undergoing physical examinations during the same period served as the control group. Serum levels of BAFF， Bb， and UACR were measured and compared between the two groups. Pearson correlation analysis was performed to assess the associations among serum BAFF， Bb， and UACR. The patients in the study group were followed up for 12 months and stratified into a favorable prognosis group and a poor prognosis group （defined as no remission） based on disease remission status. Demographic and clinical characteristics， as well as serum BAFF and Bb levels， were compared between the two outcome groups. Multivariate logistic regression analysis was conducted to identify independent predictors of poor prognosis in IMN patients. Receiver operating characteristic （ROC） curve analysis was employed to evaluate the predictive value of these indicators for poor prognosis. Results The levels of BAFF， Bb， and UACR in the research group were significantly higher than those in the control group （P < 0.05）. Pearson correlation analysis revealed that both BAFF and Bb were positively correlated with UACR （r = 0.716 and 0.543， respectively； P < 0.05）. After one year of follow?up， a total of 200 patients completed the study， among whom 46 experienced poor prognosis. Based on the follow?up outcomes， patients were categorized into a poor prognosis group （n = 46） and a good prognosis group （n = 154）. The poor prognosis group exhibited significantly higher levels of serum creatinine， 24?hour urinary protein， BAFF， and Bb， as well as lower eGFR values， compared to the good prognosis group （P < 0.05）. Multivariate regression analysis identified 24?hour urinary protein， BAFF， and Bb as independent risk factors for poor prognosis in patients with IMN， whereas eGFR was identified as a protective factor （P < 0.05）. ROC curve analysis demonstrated that the combined assessment of 24?hour urinary protein， eGFR， BAFF， and Bb yielded a higher AUC value for predicting poor prognosis in IMN patients than any single indicator alone （Z = 4.145， 3.908， 4.308， 3.864； P < 0.05）. Conclusions The serum levels of BAFF and Bb in patients with IMN are significantly elevated and positively correlated with the UACR. The combination of 24?hour urinary protein excretion and eGFR demonstrates high predictive value for poor prognosis in these patients. These findings suggest that BAFF and Bb may serve as potential biomarkers for prognostic assessment in IMN， and the integrated measurement of multiple parameters could provide a more comprehensive clinical basis for disease management.