In order to optimize the application of laser underwater cutting technology in nuclear facilities and ships and improve the cutting quality of Q235 steel plates, a method combining single factor and orthogonal experiments was adopted, while gas pressure, laser power and cutting speed were used as the main variables. And the experiment was carried out. The results show that gas pressure significantly affects the roughness of the cutting surface, and laser power has the greatest impact on the amount of slag. The optimal parameters are gas pressure 0.8 MPa, laser power 4 kW, and cutting speed 3.5 mm/s, respectively. At the same time, the cutting effect is best. Macroscopic and microscopic analysis also revealed that there is a layering phenomenon during the cutting process, which mainly includes the upper layer, the middle layer, the lower layer and the slag layer. This result will help further optimize laser cutting parameters and improve the accuracy and efficiency of underwater cutting of Q235 steel plates.