Transparent-transparent plastic laser transmission welding is one of the toughest difficulties in the field of plastic laser welding. The key to solve this problem is to use the right absorber. The laser welding of transparent-transparent polypropylene technology is studied by using three kinds of absorbers: the carbon, the black polypropylene particles and the black paint. Welds with desirable property are obtained when using black paint as the absorber. The research also investigates the influence of welding parameters (laser power, welding speed, the thickness of sample and thickness of black paint layer) on the welding quality. It turns out that the appearances of welding defects, such as porosities, thermal deformation holes and surface combustions, are related to low purity of raw materials and excessive energy input. These defects can be taken into control by using proper welding parameters and argon gas.