Objective To explore the correlation between the immune status of patients with multiple myeloma （MM） and the dynamic changes in peripheral blood red blood cell distribution width standard deviation （RDW-SD）， serum free light-chain κ/λ ratio （sFLCR）， and soluble B-cell maturation antigen （sBCMA）， as well as their implications for prognosis. This study aims to provide a reference for evaluating disease progression and assessing patient outcomes. Methods 182 MM patients admitted to the hospital between July 2019 and July 2021 were enrolled as the study group. All selected patients were followed up for 3 years， with 6 cases lost to follow-up， resulting in a final cohort of 176 patients. These patients were further divided into two groups based on their prognosis： the poor-prognosis group （53 cases） and the good-prognosis group （123 cases）. Additionally， 50 healthy volunteers who underwent health check-ups during the same period were randomly selected as the control group. The immune status of both the study group and the control group was compared. Univariate analysis was conducted to identify factors associated with poor prognosis in MM patients， and Cox regression analysis was performed to determine risk factors for poor prognosis. The good-prognosis group was designated as the negative group， while the poor-prognosis group was designated as the positive group. The predictive value of peripheral blood RDW-SD， serum sFLCR， and sBCMA-both individually and in combination-for poor prognosis in MM patients was evaluated by constructing receiver operating characteristic （ROC） curves. The area under the curve （AUC） was calculated， and the optimal cut-off value was determined using the Youden index. Finally， the predictive value of the combined test was analyzed by fitting an appropriate equation. Results Levels of peripheral blood Th17 cells and platelet-to-lymphocyte ratio （PLR） were significantly higher in the study group compared to the control group （P < 0.05）， while the level of peripheral blood regulatory T cells （Tregs） was significantly lower than that in the control group （P < 0.05）. Additionally， levels of peripheral blood RDW-SD and serum sBCMA were significantly higher in the poor prognosis group compared to the good prognosis group （P < 0.05）， whereas the serum level of sFLCR was significantly lower than that in the good prognosis group （P < 0.05）. Cox regression analysis revealed that elevated peripheral blood RDW-SD （HR = 1.091， 95%CI： 1.027 ~ 1.159）， reduced serum sFLCR （HR = 1.095， 95%CI： 1.035 ~ 1.159）， and increased serum sBCMA （HR = 1.095， 95% CI： 1.016 ~ 1.165） were independent risk factors for poor prognosis in patients with MM （P < 0.05）. ROC curve analysis demonstrated that the combination of peripheral blood RDW-SD， serum sFLCR， and sBCMA assays achieved an AUC value of 0.880 for predicting poor prognosis in MM patients， which was significantly higher than those of the three individual assays （AUC values： 0.805， 0.786， 0.780； P < 0.05）. The sensitivity and specificity of this combined assay were 94.34% and 68.29%， respectively. Conclusions MM patients exhibited abnormal immune status and poor prognosis， which was associated with elevated levels of peripheral blood RDW-SD and serum sBCMA， as well as reduced serum sFLCR. Moreover， the combination of peripheral blood RDW-SD， serum sFLCR， and sBCMA demonstrated superior predictive value for poor prognosis in MM patients.