The simultaneous high strength and high plasticity of metastable-β titanium alloy sheets remains a difficult challenge. In this work, a 1.2 mm thick sheet of Ti–15Mo–3Al-2.7Nb-0.25Si titanium alloy was obtained after rolling and annealing, and then subjected to a duplex-ageing treatment. The pre-ageing temperatures fell in the range of 300–390 °C for 0.5–2h. The re-ageing temperatures were in the range of 500–600 °C. After characterization, it was found that the alloy precipitates ω-phases during the pre-ageing process, and the distribution of such ω-phases evolves unevenly and increases in number with the increase of the pre-ageing temperature. Therefore, the strengthening mechanism of the alloy sheet is mainly α-phase nucleation assisted by ω-phase. Moreover, grain refinement appears as a mechanism to ensure the plasticity of the alloy. Finally the ω-phase evolution law is discussed in detail and the strengthening mechanism of the alloy sheet is also analyzed. At this work, a metastable-β titanium alloy sheet with strength of 1445.75 MPa and elongation of 6.55 % was obtained. This study provides a new idea for the microstructure design and process improvement of the metastable-β titanium alloy sheet.