Abstract: 2 mm thick T2 copper plate and 2Al6 Aluminum alloy have been used in laser welding docking experiment. In this paper, the microstructures of the soldering seam were analyzed by SEM and EDS, and combined with the movement characteristics of molten pool, the formation mechanism of compound in the soldering seam is explored. The results show that due to the rapid solidification of soldering seam, there is a larger temperature gradient and concentration gradient is formed in the soldering seam. Under the action of the concentration gradient diffusion and the surface tension, the convection is formed, and lead to the formation of double pool. Among them, copper side of the pool for the uniform distribution of α-Cu solid solution, the aluminum side pool for massive α-Al solid solution and mesh A1₂Cu compounds, The transition layer is consisted of two layers, the I layer is uniformly distributed Al₂Cu and the II layer is a block phase. (Al₂Cu and α-Cu). The formation of the soldering seam is essentially the suitable concentration of Al, Cu atoms diffusion and joint of solidification, the process is divided into four stages, which is the parent metal melting, the formation of double pool and weld layer group II, the grow up of weld layer group II, and the formation of double pool and weld layer group I, respectively.