Laser transmission welding of plastics based on diffraction is a new technology of simultaneous welding. This paper introduces the principle of laser transmission welding of plastics based, and tells a laser simultaneous welding technology based on diffractive optical element. Diffractive model is established and based on the weighted G-S algorithm, the influence of the size of weighted factor on the design of diffractive optical element (DOE) is numerically simulated, at the same time, the influence of the size of weighted factor on the welding is analyzed by using Matlab. The simulation results indicate that with the weighted factor increasing, the diffractive efficiency is improved that means the light intensity in the emitting region is enhanced, and meanwhile, sun of square error (SSE) is improved that means the uniformity of the spot is decreased after reshaping, and the light intensity in the emitting region may be non-uniform. Finally, the experiment of modulation and reshaping of DOE is carried out, the pre and post spot shapes are acquired, attaining the expected pattern.