In order to explore the influence of technological parameters on the cutting section quality in the process of thick plate laser cutting, the 30000 W ultra-high power fiber laser cutting machine independently developed by Penta laser was used to carry out four factors and five levels orthogonal tests on 304 stainless steel with a thickness of 40 mm, and the residual results were analyzed. The influence law of each process parameter on the cutting section index (upper and lower slit width, perpendicularity and the roughness value of the upper, middle and lower plates) was obtained. The results show that the influence of slit width on the upper surface is defocus amount, auxiliary gas pressure, laser power, and cutting speed. The influence of perpendicularity is cutting speed, defocus amount, laser power, and auxiliary gas pressure. The primary and secondary effects of roughness are as follows laser power, defocus amount, auxiliary gas pressure, and cutting speed. The results of the residual analysis show that the optimized process parameters of ultra-high power laser cutting 304 stainless steel with 40 mm thickness are as follows laser power 20000 W, cutting speed 200 mm/min, auxiliary gas pressure 18 bar, and defocus amount +11 mm.