A 5 kW disk laser is used for the butt filler wire welding of 1.1 mm thick advanced high strength steels. The laser process parameters are designed by the orthogonal test, and the tensile strength, microhardness, surface and cross section of these welded joints are analyzed by the universal mechanical test machine, microhardness tester, optical microscope and scanning electron microscope. In addition, the effects of laser process parameters on the mechanical properties and microstructures of the welded joints are studied. The results show that laser power and welding speed are the most important parameters influencing the performances of joints in the range of test parameters. Under the reasonable process parameters, that is, when the line energy is greater than the threshold value, the weld appearance with a uniform, continuous and smooth surface can be obtained, and the tensile strength is more than 90% of that of the base material. There exist for all the joints the phenomena of fusion zone hardening and lateral heat-affected zone softening. The microstructure of the fusion zone is mainly composed of lath martensite and proeutectoid ferrite. The microstructure of the heat affected zone is softened due to the emergence of large quantities of tempering martensite, which affects the tensile properties of joints.