The effects of sub-transus (α+β) annealing treatment (ST), followed by single aging (SA) or duplex aging (DA) on the microstructural evolution and mechanical properties of near-β Ti-4Al-1Sn-2Zr-5Mo-8V-2.5Cr (mass fraction, %) alloy were investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results show that the finer secondary α phase precipitates in the alloy after DA than SA (e.g., 149 nm for SA and 69 nm for DA, both after ST at 720 °C). The main reason is that the pre-aging step (300 °C) in the DA process leads to the formation of intermediate ω phase nanoparticles, which assist in the nucleation of the acicular secondary α phase precipitates. In addition, the strength of the alloy after DA is higher than that of SA at the specific ST temperature. A good combination is achieved in the alloy subjected to ST at 750 °C, followed by DA (UTS: 1450 MPa, EL: 3.87%), which is due to the precipitation of nanoscale secondary α phase by DA. In conclusion, DA is a feasible process for this new near-β titanium alloy.