Polyprogylene (PP) plastic is welded by using laser transmission welding technology. Feasibility of laser welding PP plastics is discussed. The orthogonal experiment is applied to study the influence of laser power, welding speed and mass fraction of carbon black on the welding strength and welding quality. Effect of line energy on welding strength is discussed. The results show that the laser power is the primary influence factor, followed by the welding speed, the last is the mass fraction of carbon black. The best welding parameters for PP are laser power of 50 W, welding speed of 15 mm/s and mass fraction of carbon black of 0.15%. Line energy has an impact influence on welding quality. When line energy is in the range of 1.5~3 J/mm, the higher welding quality can be attained.