The influence of B and Y on the microstructure, microsegregation, and tensile behaviour of the Ti45Al8Nb0.2W0.25Cr (at%) alloy was investigated. The β-stabilizer elements in the high-Nb TiAl alloy promote the formation of the γ phase in the microsegregation region and lead to the formation of large blocky microsegregation areas. The large blocky microsegregation regions with low specific surface areas reduce the nucleation rate of cavities and cracks at the interfaces of the microsegregation, which are harmful to colony boundary strengthening and decrease tensile resistance. The addition of B and Y affords an obvious refinement in the lamellar colony, renders an increasing opportunity for cavity nucleation at the colony boundary, and thus improves the tensile resistance. The tensile mechanisms of the alloys before and after (B, Y) addition were also compared and analysed.