Topological phase transitions have attracted significant attention recently. (Bi_1-xSb_x)₂Se₃ demonstrates a metal-insulator transition as well as a topological phase transition from a topological insulator to a normal insulator driven by isoelectronic substitution. Although this topological phase transition has been studied both theoretically and experimentally for thin films and single crystals, very conflicting results have been reported and the critical transition point is still under debate. The symmetric study of surface electronic symmetry has never been conducted. Rotational second-harmonic generation (RSHG) is a powerful and non-invasive nonlinear optical technique for probing the electronic symmetry originating from broken symmetry at the surface. In this talk, we symmetrically characterize the surface electronic symmetry and the topological phase transition using high-quality (Bi_1-xSb_x)₂Se₃ single crystals. From the RSHG patterns, we confirm the Bi₂Se₃ surface shows a C_3v symmetry and the Sb₂Se₃ surface shows a C_2v symmetry. By analyzing the evolution of RSHG patterns with substitution concentration, the critical transition point is further derived. The interplay between the topological phase transition and the metal-insulator transition is also discussed.