Objective:To discuss the clinical manifestations and laboratory examination results of agranulocytosis and reactive plasmacytosis(RP)in the patient with Graves'disease(GD)after treated with methimazole(MMI),and to provide the basis for the clinicians to differentiate RP from multiple myeloma(MM).Methods:The clinical manifestations,laboratory examinations,diagnosis and treatment processes of one patient with GD agranulocytosis complicated with RP were analyzed,and the related literatures were reviewed.Results:The patient had a history of GD and abdominal infection.Upon admission,a complete blood count revealed a significant decrease in white blood cell count accompanied by neutropenia,and a smear re-examination showed suspicious plasma cells.The bone marrow cytology examination results showed the percentage of bone marrow plasma cells was 33％,and the percentage of plasma cells in peripheral blood was 4％;the serum immunoglobulin results showed polyclonal hyperplasia;the serum immunofixation electrophoresis results were negative;the flow cytometry analysis results indicated the immunophenotype of the plasma cells was normal.Based on the medical history and laboratory results,MM was largely excluded,supporting the diagnosis of RP.Neutropenia was considered to be related to medication,so MMI was discontinued,granulocyte colony-stimulating factor was administered to increase the number of white blood cells,and specialized GD treatment was conducted after controlling the abdominal infection.The patient had a good prognosis,and his blood count was normal upon re-examination 6 months later.Conclusion:Agranulocytosis complicated with RP in the GD patients is clinically rare.Serum immunofixation electrophoresis,blood cell morphology,and cell immunophenotype analysis are helpful for the accurate diagnosis.After actively treating the primary disease causing RP,the patient's prognosis is favorable.