To figure out the elements distribution in the seams, laser welding was carried out on the Al plates which coating with Si powder and Cu powder respectively, then electronic probe was used to analyze the microstructure in the section of the seams. Results indicated that alloying elements distribution in the weld is heterogeneous under the metal liquid movement in welding pool and the elemental diffusion co-functioning. There are two high concentration regions on both sides of the weld centerline, the content at the center is low, slightly higher and more homogeneous at the bottom. The distribution of the alloy composition in the weld is related to the specific gravity of the alloying elements. Though the specific gravity of Si and aluminum matrix is similar, it can flow with the liquid metal in the pool, so the concentration changes of Si in the weld are larger. Elemental fluctuation keeps steady in the weld joint.