Laser nitriding is needed to improve the surface hardness and wear resistance of titanium alloys for nuclear power equipment.There are many procedure factors affecting the semiconductor laser nitriding of titanium alloys,defocus distance is one of the key factors. The effect of defocus distance on the semiconductor laser nitriding of TC4 titanium alloy is studied. Other procedure parameters remain unchanged,the laser nitriding tests of TC4 titanium alloy is carried out by using 5 mm,10 mm,15 mm and 20 mm defocus distance respectively,the surface and cross-section appearance of nitriding layer are analyzed,the hardness of nitriding layer is measured,the microstructure of nitriding ed layer is photographed,the contrast friction test of nitriding sample and TC4 matrix is conducted. The results show that semiconductor laser can nitride on the surface of TC4 titanium alloy,when the defocus distance increases,the size and hardness of nitriding layer decrease,and the comprehensive performance deteriorate. Under the experimental condition of this study,the comprehensive performance of laser nitriding layer by 5 mm defocus distance is optimal,its maximum nitriding layer thickness is 610 μm,the nitriding layer has the characteristic golden color of TiN,the maximum surface hardness of nitriding layer can reach 982 HV,which is significantly higher than of TC4 substrate（350 HV）,the wear test under the same conditions results in a mass loss of 0.2 mg for the nitriding sample and 1.78 mg for the TC4 substrate.