Abstract: This paper investigates the microstructure evolution and performance characteristics of the heat-affected zone (HAZ) in 10Ni5CrMoV steel laser welding. By designing novel thermal simulation specimens and utilizing the GLEEBLE 3500 testing machine, the thermal cycle behavior during laser welding was simulated. The peak temperatures (Tm) were 1350, 1000, and 765 ℃respectively, with t_8/5 times of 1.5, 2.5, and 5.0 s. The research employed multiple characterization methods including impact testing, tensile testing, metallographic analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). The results indicate that when Tm was 1350 ℃ and 1000 ℃ respectively, the primary microstructure under different t_8/5 times was lath martensite, and the grain size increased significantly with the increase of peak temperature. When Tm = 1350 ℃ and t_8/5 = 1.5 s, the impact energy significantly decreased to 43 J. The analysis suggests that this was due to the formation of large carbide particles between martensite laths, leading to a reduction in material toughness. However, under the same peak temperature, when t_8/5 was extended to 2.5 s, the impact energy was comparable to that of the base metal. This was primarily attributed to the synergistic effect of lath martensite and retained austenite, which significantly enhanced the material toughness.