To investigate the physical principle of laser transmission welding of plastics, comprehend the effect of welding parameters on the welding quality, find out the optimum welding parameters and improve the quality of plastic transmission welding, an experimental investigation of transmission welding of polymethyl methacrylate (PMMA) with 1064 nm fiber laser is carried out on the fiber laser material processing system. The physical phenomena in the welding process are analyzed, the welding quality for different welding parameters such as laser power, laser diameter and welding speed is compared. The influencing law of welding parameters on the welding quality and the optimum welding parameters under special condition are found out. The welding piece is tested by a pull testing unit. The result shows that the strength of the welding seam is higher than that of base material.