25 mm thick 27SiMn high strength steels are welded with the fiber laser-metal active gas (MAG) hybrid welding method, and the microstructures and mechanical properties of the welding joints are analyzed. The results show that the weld seam and heat affected zone of the root-face pass of the joints are mainly composed of martensite. The weld seam of the filler pass mainly consists of intragranular acicular ferrite, a small amount of pre-eutectoid ferrite and upper bainite, and the microstructure of the heat affected zone is mainly martensite. The microhardness distribution of welding joints is not uniform and the highest microhardness of the root-face pass is at the weld seam. The highest microhardness of the filler pass is in the coarse grain area, and the lowest microhardness is in the base metal. Tensile specimens are all broken in base metals. The impact experimental results show that the brittle fracture zone and toughness fracture zone exist in the impacted fracture of the weld at the root-face pass, while the toughness fracture zone is observed in the impacted fracture of the weld at the filler pass.