A new type of near-β titanium alloy (Ti555211) was investigated. This alloy has excellent plasticity, high specific strength, and excellent comprehensive properties, which is widely used in the aerospace and chemistry industries. Through the 3×3 orthogonal experiments, the influences of different stages of two-step annealing treatment (solution temperature, aging temperature, aging time) on the mechanical properties and microstructures of Ti555211 titanium alloy were investigated. Results show that with increasing the solution temperature and decreasing the aging temperature, the alloy strength is increased. The elongation is increased with decreasing the solution temperature and increasing the aging temperature. After treatment of 820 °C/2 h/air cooling and 580 °C/12 h/air cooling, the alloy has better plasticity, and its tensile strength reaches 1333 MPa, which is higher than the strength index (1080 MPa) of similar alloys by 20%. The elongation of the Ti555211 titanium alloy is 12%, which is higher than the plasticity index (5%) of similar alloys by 140%.