In order to improve mechanical properties of 1Cr5Mo heat-resistant steel welded joints, the surface was modified with CO2 laser, and the tensile tests were conducted to research the effect of LHT(laser heat treatment) on tensile properties of welded joints. The rupture modes and the fracture morphologies were analyzed with SEM(scanning electron microscopy), the changes of chemical elements in the sample surface before and after LHT were analyzed with EDS(energy disperse spectroscopy), and the effects of residual stress and retained austenite on tensile properties were discussed. The results show that the fracture of the samples before and after LHT are ductile rupture, the rupture properties have been improved. After LHT, the strengthened layer of compressive residual stress and refined grains are generated in the sufrace, which cause yield strength and tensile strength to increase by 3.4% and 13.7%, and the tensile properties increased. The content of retained austenite decreases in the sample surface after LHT, which causes the section reduction rate to decrease by 18.61%, the elongation rate nearly doesn′t change, and the ability of uniform deformation has improved.