Background . Myelodysplastic syndromes (MDS) are clonal hematopoietic stem cell disorders characterized by bone marrow dysplasia, ineffective hematopoiesis, and cytopenias. Monoclonal gammopathy of undetermined significance (MGUS) and multiple myeloma (MM) patients have a high risk of secondary MDS or acute myeloid leukemia (AML) compared to healthy persons, and chemotherapy or transplantation may result in secondary treatment-related MDS. Methods . A patient was diagnosed with both MDS and MGUS, which was treated using thalidomide, dexamethasone, and danazol. A follow-up blood test was conducted to determine leukocyte and hemoglobin levels. Results . Immunoprotein electrophoresis showed M protein peak with IgA+ kappa components. Nuclear cells proliferated actively in bone marrow aspirates. Bone marrow analysis suggested a myelodysplastic syndrome with myeloblastoma (MDS-RS) and a new plasmacytoma. The immunophenotype was shown as follows: R5 cells (red) are about 15.5%. Among the CD38+CD45 cells, about 95.9% of cKappa cells and 1.7% of cLambda cells are considered as plasmacytoma. Gene detection showed that the patient carried 14 gene mutations, and karyotype analysis showed that they had normal male chromosome structure. The patient was diagnosed as MDS and MGUS, and finally discharged after treatment with thalidomide (75 mg daily), dexamethasone (3 mg daily), and danazol (200 mg twice daily). Within 1 year, the disease has stabilized. Conclusion . The combination of plasma cell disease and myeloid malignancy may increase mortality. This is uncommon and may be easily misdiagnosed if not detected early. When a myeloid neoplasm tests positive for MDS and serum M protein, clinicians should evaluate for other plasma cell disease