The effect of in situ reaction on the microstructure of Nd:YAG laser welded joints of aluminum matrix composite SiCp/AlSi7Mg was studied. Results showed that the laser welding with Ti filler improved the tensile strength of welded joints. Moreover, the laser welding with in situ reaction effectively restrained the pernicious Al4C3 forming reaction in the interface between aluminum matrix and reinforcement particles. Simultaneously, the reaction-formed TiC phase distributed uniformly in the weld. This permitted SiCp/AlSi7Mg composite to be successfully welded by Nd:YAG laser.