In order to understand the influence of the different aging parameters on the microstructure and mechanical properties of Ti-6Al-3Nb-2Zr-Mo (Ti6321) alloy, the microstructures and mechanical properties under different aging parameters (temperatures ranging from 500 °C to 650 °C, 3–24 h) were investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and mechanical properties tests. The results reveal that the secondary α phase (α_s) is more sensitive to the aging parameters than the primary α phase (α_p). Moreover, the thickness of α_s phase is positively correlated with aging temperature or aging time. As the aging temperature and aging time increase, the segregation of Ti and Al elements in the β transformation phase (β_t) becomes obvious, and the α_s phase changes from fine needle-like to long rod-like. When the alloy is aged at 600 °C for 12 h, the alloy shows good comprehensive mechanical performance. The tensile strength, yield strength, and elongation are 907 MPa, 796 MPa, and 16%, respectively, and the impact energy is 55 J.