Abstract: Laser cutting of thick plates were carried out by simulating the circumstance of underwater 50 m. The influencing factors such as oxygen pressure, distance from nozzle to workpiece surface, cutting speed were researched and the results were evaluated by cutting widths and roughness of fracture surfaces. The results indicated that the bottom of the plate was difficult to cut through when oxygen pressure was low, with slag connected at the bottom. The deslagging ability increased with the increase of the oxygen pressure, results in the increasing of kerf flatness. In underwater laser cutting, fracture surface of laser cut steel become bright and clean as oxygen pressure increased, differing from the result performed in the atmosphere. No obvious difference was observed when it came to the variation of distance from nozzle to workpiece surface on cutting quality, 2-4 mm was employed in this study. As the cutting speed increased, the reduction of laser cut width in the bottom was most obvious and cleavage lines of laser cut became curved. When the speed reached to 2.0 m/min, fracture surface of laser cut steel was observed to be strip-like type.