Laser plastic welding could improved the quality of products greatly, so the demand of laser plastics welding would be more and more. There are many process parameters affecting plastic welding, and lots of experiments such as orthogonal experiment were required to test for getting the best process parameters. In this paper, laser transmission welding testing for thermoplastic ABS plastic was carried out. Single factor experiments of laser power, welding speed and focal distances were used to analyze the regular of welding effects. Then power density was used to research shear strength and microcosmic. When power density was less or more than 1.6 W/mm2, the welds was brittle fracture, shear strength was weak. When power density was 1.6 W/mm2, the welds was ductile fracture, and shear strength was reached to the highest of 71 MPa. The analysis results showed that, single factor of laser power density had direct effect on the microcosmic of welds and then effect on the shear strength.