The process of laser transmission welding of polycarbonate is studied. The relationship between welding quality and surface roughness is investigated from the perspectives of fracture morphology and material absorptivity. Based on the Weierstrass-Mandelbrot function and measured points data, the three-dimensional real surface model is built and the temperature field numerical simulation of single-board welding is carried out. Experimental results show that, with the increase of the specimen surface roughness, the welding quality deteriorates. Simulation results show that the rough surface reaches peak temperature faster than the smooth surface, while the peak temperature of the smooth surface is higher than that of the rough surface. With the increase of the welding rate, the welding temperature appears an overall decline, and the temperature variation curves of surfaces with different roughness overlap gradually.