Objective: To analyze the spectrum of abnormal serum free light chain ratio (sFLC kappa/lambda ratio), and to redefine the range of sFLC kappa/lambda ratio, so as to achieve hierarchical diagnosis of diseases with abnormal sFLC kappa/lambda ratio, resulting in the increased sensitivity and specificity in the diagnosis of monoclonal plasma diseases. Methods: Enrolled 1,340 patients with abnormal sFLC kappa/lambda ratio (< 0.26 or > 1.65) were grouped: (1) group A: malignant plasma diseases; (2) group B: monoclonal gammopathies of undetermined significance (MGUS); (3) group C: reactive plasma diseases. These patients were further divided by renal function eGFR < 60 or > 60 ml/min/1.73m(2) to eliminate renal diseases influencing the results. Statistical analyses was performed by using SPSS 22 software. Results: When sFLC kappa/lambda ratio > 3.49 and eGFR > 60ml/min/1.73m(2), the sensitivity and specificity of the diagnosis of malignant plasma diseases were 86.1% and 94.0%, respectively. When sFLC kappa/lambda ratio > 2.89 and eGFR < 60ml/min/1.73m(2), the sensitivity and specificity of the diagnosis of malignant plasma diseases were 92.0% and 97.0%, respectively. Conclusion: The sensitivity and specificity of the diagnosis of monoclonal plasma diseases can be significantly improved by redefining the cut-off value of sFLC kappa/lambda ratio and the renal function index of eGFR.