The mechanical property tests of near β-type Ti55531 titanium alloy after solution (820 _^oC/2 h) plus aging (580~640 _^oC/6~10 h) treatments were carried out on INSTRON-5948R micro tensile machine. The effects of solution plus aging treatment on microstructural evolution and mechanical properties of Ti55531 alloy were studied to obtain a better combination of ultimate tensile strength and ductility. Furthermore, the fracture mechanism of the alloy during tensile test was discussed. The results show that the secondary lamellar α phase is more sensitive to the change of aging parameters than the primary α phase. and the thickness of lamellar α phase is linearly positively correlated with aging temperature or time. Compared with aging time, the coarsening rate of the secondary lamellar α phase is less sensitive to aging temperature, and the coarsening rate of the secondary lamellar α phase with aging temperature and aging time is about 1 nm/oC and 8 nm/h, respectively. After solution and aging treatment, the mechanical properties of Ti55531 alloy are significantly improved, which reach the best comprehensive mechanical properties with solution treatment at 800 _^oC for 2 h plus aging at 640 _^oC for 8 h. Under this condition, the ultimate strength is 1144 MPa, the elongation is 8.16%, and the strength-ductility product exceeds 9.3 GPa .%. The tensile fracture modes of Ti55531 alloy are ductile and brittle mixed fracture, and mainly ductile fracture, including intergranular cracking and microvoid coalescence mechanisms.