Abstract: Langerhans cell histiocytosis(LCH)and Langerhans cell sarcoma(LCS)are characterized by clone proliferation of Langerhans-type cells, which may occur concurrently or sequentially with T-cell acute lymphoblastic leukemia (T-ALL) and other Lymphoid neoplasms. A 15-year old female patient diagnosed with T-ALL developed LCH involving multiple systems during maintenance chemotherapy of T-AL. After treated with chemotherapy with improved result, the patient showed progression of the illness and refractory to the second-line treatment. We found c.G35A (p.G12D)mutation in the KRAS gene and used the targeted drug Trametinib for treatment. The treatment proved effective, leading to partial remission within a week. Three months after Trametinib treatment, the patient developed new lymphadenopathy. Biopsy revealed the existence of LCS. The disease progressed quickly, and the patient died 7 days after diagnosis of LCS. The case of patients with T-ALL then developing LCH and LCS sequentially is extraordinarily rare. The causes of the case is unclear and may be related to cell transdifferentiation, clonal evolution, and chemotherapy. Targeted drugs can contain this disease for a short time.