The invention provides a reagent composition for screening and/or diagnosing clonal diseases by a one-step method and application of the reagent composition. The reagent composition comprises eight groups of antibodies and is a flow cytometry detection composition capable of being used for one-step screening and/or diagnosis of clonal diseases, a five-tube parallel scheme is used during application, the first group of antibodies and the second group of antibodies are respectively used for different tube samples, the third group of antibodies and the sixth group of antibodies are used for the same tube sample, the fourth group of antibodies and the seventh group of antibodies are used for the same tube sample, and the fifth group of antibodies and the eighth group of antibodies are used for the same tube sample. The reagent composition disclosed by the invention can be applied to flow cytometry one-step screening/diagnosis of various clonal diseases and other abnormalities, comprising acute leukemia, mature lymphocyte tumor, chronic myeloid tumor, night paroxysmal sleep hemoglobinuria, metastatic cancer, precancerous lesions, rare normal human polymorphisms, aplastic anemia, other immune cell subset abnormalities and the like.