Acute myeloid leukemia (AML) is a myeloid malignant hematological disorder. Hairy cell leukemia (HCL) is a rare B-lymphocyte proliferative disorder. Co-existence of AML and HCL in a same patient is rare. The development of AML after CHL treatment, especial purine analogs, such as pentostatin (2’-deoxycoformycin) and cladribine, has been described in 14 cases. While the synchronous development of AML and CHL in a same patient was report only in one case. Here we reported another one. This is a 78-year-old man who presented with pancytopenia. Bone marrow smear showed 58% myeloblasts and 15% hairy cells. Immunophenotype of bone marrow mononuclear cells confirmed two groups. Cells of group A were CD34++, CD117++, CD33++, CD13++, CD123++, HLA-DR++, and CD7+, while cells of group B were CD123++, CD22++, CD20++, CD19++, cCD79a++, CD11c++, CD103++, CD25++, cKappa+, and HLA-DR+. By exon sequencing typical mutations, including SRSF2P95H, BRAFV600E, IDH1R132C, ASXL1Y591X, TET2 T1983Nfs*30, NRAS K42R, and NOTCH3 P167S, were detected. Thus the patient was diagnosed as AML and HCL. We reported a very rare case with synchronous development of AML and HCL.