T-cell acute lymphoblastic leukemia (T-ALL) is the hematological malignancy of bone marrow characterized by the rapid proliferation and subsequent accumulation of immature T lymphocyte and mainly occurs in children and adolescents. In 1991, a kind of activating mutation of Notch 1 was found in a subset of T-ALL with chromosomal translocation t(7;9) for the first time. During the past 20 years since then, understanding of the relationship between Notch 1 activating mutation and T-ALL has been deepened and widened. This review briefly discusses the four main subtypes of Notch 1 activating mutations, also focuses on how these mutations change the normal signaling pathways and genes expression during their participation in the pathogenesis of T-ALL, and how these insights will promote the development of newly targeting therapies for patients with this aggressive form of leukemia.