Herein, an FV520B steel joint is formed through laser backing welding and cold metal transfer (CMT) filler welding. Microstructures and properties of the joints at different processing parameters are studied. Microstructures of the fusion zone (FZ) obtained using laser backing welding mainly comprise lath martensites, which are arranged in a parallel form in the primary austenite grains, and δ ferrites, which are situated at the primary austenite grain boundaries and lath martensite interfaces. Some continuous and discrete δ ferrites are located at the fusion line of laser welding. The weld width, weld penetration, and the width of heat-affected zone (HAZ) increase with the increase of the heat input of the CMT filler welding. Moreover, the microstructural characteristics of the HAZ and FZ of laser backing welding gradually disappear. When the heat input of CMT filler welding is lower, the grains in the heat-affected laser welding fusion zone (HALWFZ) close to the filler welding fusion line exhibit an equiaxed shape because of the high temperature reheating. Furthermore, the size of the equiaxed grains decreases with the increase of distance to the filler welding fusion line. When the heat input of CMT filler welding is higher, the columnar grain microstructures in the laser welding fusion zone transfer into the larger equiaxed grains. Compared with the single laser welding, the hardness of the weld cross-section exhibits more uniform distribution in the horizontal direction after the filler welding. With the increase of heat input of filler welding, the average hardness of HALWFZ initially increases and then decreases. The average hardness of the filler welding fusion zone (FWFZ) is lower than that of the laser welding fusion zone (LWFZ). Moreover, the average hardness of the LWFZ presents the lowest value in the vicinity of the filler welding fusion line. After filler welding, the strength of laser backing welding area is higher than that of the base metal, whereas the strength of filler welding area is lower than that of the base metal. The impact toughness of laser backing welding area increases with the heat input of filler welding. Electrochemical corrosion results show that with the increase in the heat input, the corrosion potential of LWFZ initially increases and then decreases. The LWFZs before and after filler welding both exhibit a superior corrosion resistance than the base-metal substrate.