Light scattering of the upper polymer have a great influence on welding quality. Light scattering of high density polyethylene (HDPE) and low density polyethylene (LDPE) are assessed by constructing experiment and numerical computation method. Firstly, the beam quality of semiconductor laser is analyzed, power flux distribution of the laser beam in a defocused plane is measured by knife edge method; Afterwards, the power flux distributions of the laser beam after passing through HDPE/LDPE are measured by line scanning method; Lastly, with the combination of the mathematical model which is used to calculate scattering coefficient and standard deviation of scattering, scattering related parameters and the laser power flux distribution at the welding interface are obtained by writing a program in MATLAB. The results show that the light scattering coefficient of high density polyethylene is up to 0.988, the light scattering coefficient of low density polyethylene is 0.92; Higher crystalline polyethylene leads to more obvious light scattering; the laser beam power flux distribution at the weld interface affected by scattering is determined, which lays a solid foundation on numerical simulation in laser transmission welding.