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SrCd₂As₂ is the layered analogue of the prototypical Dirac semimetal Cd₃As₂, yet it is a trivial insulator. Recent theoretical work suggested that the negative pressure induces a band inversion in SrCd₂As₂ and stabilizes the topological Dirac semimetal phase. In this work, we successfully grew SrCd₂(As_1-xSb_x)₂ single crystals, where Sb substitution generates a negative chemical pressure. As the Sb doping level increasing, the resistivity changes from insulating to metallic behavior. For high Sb concentration, we observed pronounced single-frequency Shubnikov-de Haas oscillations. These oscillations correspond to a small hole pocket with an extremely light effective mass 0.05 m_*, consistent with the existence of Dirac dispersion. Therefore, we argued that SrCd₂(As_1-xSb_x)₂ exhibits a topological phase transition from a trivial insulator SrCd₂As₂ to a topological Dirac semimetal, and the material offers a platform to understand the evolution of non-trivial electronic band topology.