Microstructure evolution and mechanical properties of Ti-1023 alloy during cooling from single-phase region to dual-phase region with the cooling rate of 1 °C/min were systematically investigated. As the final cooling temperature decreased, grain boundary α'''' martensite (α''''_GB), grain boundary α phase (α_GB), Widmanstatten grain boundary α phase (α_WGB), and Widmanstatten intragranular α phase (α_WI) were sequentially precipitated. Correspondingly, mechanical properties and deformation mechanisms of Ti-1023 alloy changed significantly. When the final cooling temperature was higher than 700 °C, tensile curve showed an obvious double-yielding phenomenon with elongation >20%. Whereas the final cooling temperature was lower than 700 °C, and the double-yielding phenomenon disappeared. This was mainly attributed to the fact that β stability of Ti-1023 alloys was enhanced with the increase of α(α'''') precipitated phases. And the room temperature deformation mechanism gradually changed from stress-induced α" martensitic phase transformation to dislocation slip. Furthermore, strengthening mechanism of Ti-1023 alloy was analyzed in combination with grain size and precipitated phase effects.