With the experiment of laser welding of X100 pipeline steels, the influence of heat input on the microstructure and hardness of welded joints is studied, and the mechanical properties of typical samples with full penetration are analyzed. The study results show that the full penetration welds can be obtained when the heat input reaches 1.5 kJ·cm-1 and the corresponding welding width and heat-affected zone width reach 1.8 mm and 1.0 mm, respectively. In a certain range of heat inputs, the influence of heat input on the microstructures of welded joints in each micro-area is not obvious. The microhardnesses of the weld zone, the coarse grain zone and the fine grain zone are all higher than that of the base metal, while the microhardness of the mixed zone is lower than that of the base metal. The tensile fracture occurs in the base metal. The impact energy of welded joints with full penetration reaches the level of the base metal, and the impact energy of the heat affected zone reaches 66% of that of the base metal, all of which are typical ductile fractures.