The fit up precision is one of the facts that restrict the applications of the laser welding of the vehicle body components in a butt joint. An experimental study on gap margin in laser butt welding of 1.2 mm thick high-strength automobile steel DP800 is performed with hot wire filler under various butt gaps. Sound weld beads are obtained by optimizing welding parameters when the butt gap is not more than 100% of the steel sheet thickness (1.2 mm). The chemical composition distributions, metallographic phase and mechanical property of the weld are analyzed after welding. The result shows that the butt gaps have little influence on the chemical composition distributions and metallographic phase of the weld; the tensile strength of the weld is higher than that of the bass metal when the butt gap is not more than 83% of the steel sheet thickness (1.0 mm), while it reaches 89.9% of the tensile strength of the base metal when the butt gap is 100% of the steel sheet thickness (1.2 mm). The requirement of high fit up precision of butt joint can be loosened dramatically using laser welding with hot wire.