Laser butt welding of 1.6-mm thick dual-phase high-strength steel B340/590DP is performed by using a 4-kW fiber laser. Weld topography and cross secsion of different butt joint gaps are analyzed. And mechanical properties, microstructure and microhardness of weld are tested. Laser butt joint welding of a three-dimensional body part under optimized process is carried out. Crash tests are performed after welding experiments. The results of laser welding experiments prove that weld with good appearance and narrow heat affected zone (HAZ) can be obtained when butt joint gap is narrower than 18% (about 57.6% of focused spot diameter) of sheet thickness under the proper welding condition. When butt joint gap is narrower than 16% (about 51.2% of focused spot diameter) of sheet thickness, the tensile strength of weld is better than the base metal. Different butt joint gaps have no significant effect on the micro-hardness and microstructure of the weld. The crash tests prove that crash strength of laser welds is in accordance with the design standards. Fiber laser butt welding is better than arc butt welding for a three-dimensional body part.