[en] Highlights: • The TTT diagrams of Ti55531 with different preset microstructure show different phase transformation behaviors. • The ‘β-quenched’ alloy has highest nose temperature and shortest incubation time in secondary α precipitation. • The sequence of overall transformation rate of α precipitation varies with temperature. • The α_lamellar → β shows lower starting temperature, shorter incubation time, and faster overall rate than α_equiaxed → β. The different β ↔ α phase transformation behaviors in Ti-55531 alloys with three types of preset microstructure were studied at isothermal conditions in the temperature range of 450–750 °C. The dilatometry was employed to record the length change of the specimen. TTT diagrams for alloys with different microstructure were plotted based on the phase fractions of transformed α at different temperatures. With the help of Johnson–Mehl–Avrami theory, the calculated kinetic curves were in good agreement with experimental results. The β → secondary α transformation in the alloy with preset ‘β-quenched’ microstructure took place at higher nose temperature with a shorter incubation time in comparison with others, but exhibited lower steady transformation rate than that with preset ‘β + α_equiaxed’ microstructure. With local composition analyses, the high nose temperature was interpreted by the difference in β stability and supersaturation. The alloy with preset ‘β + α_equiaxed’ microstructure showed higher transformation rate than that with preset ‘β + α_lamellar’ microstructure, which might be associated with the richness of defects.