Abstract: The reflectance of 30CrMnSiA carbon steel under laser irradiation in 20 Pa air, 1 000 Pa air, 1 000 Pa oxygen gas and 105 Pa air was obtained by integrating sphere absolute measurement method, and corresponding temperature history curves were got. The results show that the initial reflectance of material in air increases with the pressure varying from 20 Pa to 105 Pa. The change of reflectance is slower and the inflexion temperature of reflectance change is higher in oxygen-deficient environment than those in oxygen-enriched environment. Additionally, in oxygen-enriched environment, the rapid oxidation reaction of material after heating is in favor of laser energy absorption. For given temperature of material, the reflectance of material is different in different irradiation environments.