TC4 titanium alloy specimens were prepared by laser directed energy depositon in different oxygen contents of inert atmosphere. The effects of oxygen content in the argon shielding gas on microstructure, fracture surface and mechanical properties of TC4 titanium alloy by laser directed energy depositon have been investigated by optical microstructure (OM), scanning electron microscopy (SEM) , transmission electron microscope (TEM), microhardness testing and tensile testing. The results show that the oxidation degree of the samples surface increase, and the surface color of TC4 titanium alloy samples gradually change from silver white to yellow, blue and dark gray with the increasing of the oxygen content of the atmosphere. From XRD analysis, the types of oxides on the sample surface increase, and the thickness of the oxide layers also increase gradually. The microstructure of the specimens gradually change from basket structure to acicular martensite α" structure, the laths are coarsened and the aspect ratios are reduced. The hardness and tensile strength at room temperature of TC4 titanium alloy samples increase gradually with the increase of oxygen content in the forming atmosphere. Due to the influence of metal lattice distortion and cooling rate, the tensile strength of the deposition samples increase from 920 MPa to 982 MPa and the elongation decreased from 12.4% to 10.9%.