The properties of Ti–Al–Nb alloys strongly depend on alloy compositions and microstructures. Proper design of these important aspects requires detailed knowledge of the solidification sequences and liquidus projection. However, there are still some conflicts between previous experimental and calculated results about the liquidus projections in the composition range of Ti-(40–60)Al-(5–30)Nb and this has hindered Ti–Al–Nb alloy development. In this present work, we explicitly investigated the as-cast microstructures and phase constitution of 50 alloys with the composition range of Ti-(40–60)Al-(5–30)Nb and clarified the solidification sequences. We re-constructed the partial liquidus projection and found that the primary β phase region extend to just below 55 at.% Al content and the primary α phase region extend to just below 24 at.% Nb content. Moreover, it was observed that the invariant reaction between Liquid, β, α and γ phases is a peritectic reaction (Liquid+β+γ→α) and all the Liquid+β+γ, Liquid+β+α and Liquid+α+γ three-phase regions are of peritectic type. The present work provides essential information necessary for optimizing the Ti–Al–Nb thermodynamic database to accelerate alloy development.