TiAl alloy has been widely used in the aerospace and automotive industry because of its excellent properties, but the shortcomings such as brittleness and fracture toughness are still significant at room temperature, which limits its applications. The purpose of this paper is to explain the effects of Al content on the mechanical properties and to discuss the crack propagation mechanism of single crystal TiAl using molecular dynamics simulation. The stress–strain curves and evolution process of the defects were obtained under different Al contents. The results confirmed that the Al content would affect the elastic modulus and the strength of the TiAl alloy. Because of defects, such as stacking faults, dislocations and vacancies, and their migrations result in the plasticity of TiAl tended to be better when the Al content below 45at%. And the specimen implies brittleness with the Al content greater than 45at% obviously. Furthermore, the plasticity deformation dominated the main mode in the low content, while the crack fracture presented brittleness when the Al content is high. Al contents would affect the mechanical proprieties of TiAl alloy and the plasticity of TiAl would increase with decreasing Al content. In addition, the mechanism of crack propagation would vary with the change of Al content.