TiAl alloys have become one of the most promising high-temperature lightweight structural materials in aerospace and other fields because of low density, high specific strength, high-temperature oxidation resistance and other properties. However, due to the brittleness of them, it is easy to introduce micro-cracks, holes and other defects in the forming process, which seriously affect the mechanical properties. For this reason, supersonic fine particle bombardment (SFPB), one of the surface modification techniques, was used to investigate the effect of twin boundaries (TBs) on the mechanical properties and deformation behaviour of TiAl alloys. The findings demonstrate that when the number of TBs increases, the yield strength of models with various numbers of TBs falls. The closer the location of TB to the upper surface of model, the lower the yield strength of the material. As the number of TBs increases, the obstruction of dislocation movement by TBs becomes more evident and the degree of plastic deformation of the surface of the model after bombardment becomes greater, making the material more susceptible to fracture. The closer the TB to the upper surface of the material, the more evident the inhibition of dislocation growth by the twin, which in turn affects the strength of the material. Deformation failure of the model is the combination result between dislocations and dislocations, dislocations and twins and other defects.