The influence of Mo doping on the mechanical properties of TiAl alloy has been studied by the first-principles method based on density functional theory. According to the calculated lattice parameters, elastic constants, bulk modulus and shear modulus of the systems with different doping concentrations, we find that Mo doping can improve the ductility of TiAl alloy. The strong s-s, p-p and d-d electron interactions happen among all the s-, p- and d-electrons of Ti atom and near Mo atom, which astrict effectively the migration of Ti and Al atom and is beneficial to enhance the stability and strength of the alloy.