The 2.8 mm thick 800 MPa grade hot-rolled high strength steel plates are welded with fiber laser. The fully-penetration laser welded joints are obtained by adjusting the laser power. The microstructures of laser weld joints are observed, and the microhardness, tensile properties and impact toughness of the laser welded joints are tested. The relationship between microstructure and mechanical properties of laser welded joints is studied. The results show that there was no softening zone in the welded joint, the tensile strength of the welded joint can reach that of the base metal, and the impact energy of the welded joint can reach 85.6% of the base metal.