Abstract: The microstructure of ultra high strength DP1000 steel laser welded joint was observed by optical microscope (OM) and scanning electron microscope (SEM). The mechanical properties of different heat input welding joints were studied by microhardness test and tensile test. The results show that the microstructure of the softening zone, which is composed of the tempering zone and the two phase zone, changes obviously with the increase of heat input. Furthermore, the average hardness in the softening zone decreases and the width size increases, resulting in the change of the tensile specimen fracture position. When the heat input is not higher than 52 J/mm, the tensile strength of the welded specimen is 97.75% of the base metal, the maximum width of the softening zone is about 506 μm, and the fracture occurs on the base metal. When the heat input reaches 72 J/mm, the width of the softening zone is about 621 μm, and the fracture occurs in the softening area.