In this paper, changes in the morphology and composition of the O, α2 and B2/β phases and the transformation relationship between the three phases of Ti22Al25Nb alloy were systematically investigated by performing a series of short heat treatments. The results showed that the phase transformations can be divided into three stages in the temperature range of 870 °C ~ 1080 °C. In the first stage of 870 °C ~ 930 °C, the O phase started transferring to α2 + B2 first along grain boundaries with lamellar morphology due to short-range diffusion, and the O → α2 + B2 reaction was accompanied by a large amount of Nb segregation. With the increase in temperature, the phase transformation migrated from the boundaries to the inside of the grains, and the α2-phase spheroidized gradually. In the second stage from 960 °C to 1020 °C, granular α2 and B2 phases precipitated directly from the O phase due to long-range diffusion. At 1020 °C, the O → α2 + B2 transition was completed and the granular α2 phase mainly populated along the grain boundaries. In the third stage above 1020 °C, the main phase transformation was α2 → B2, and the aggregation and growth of the α2 phase were observed in α2-rich regions.