The single laser technology was used to weld 5-mm thick Q345B low-alloy steel and 304 stainless steel. By modifying laser power, the microstructure, microhardness, and tensile strength of welded joints made of dissimilar steel were analyzed under various process parameters. The results showed that the weld could not penetrate entirely when the laser power was less than 4 kW and the weld would collapse when the laser power was greater than 7 kW. The weld microstructure was typical strip martensite when the laser power was increased from 4 kW to 7 kW; the weld center hardness was maximum and the weld porosity was the lowest at 6 kW. The samples' tensile fracture sites were all present on the Q345B base material with no weld fracture. γ-Fe,α-Fe, Cr₂Ni₃, and Fe₂Ni₃ phases dominate in welds, with little M₂₃C₆, σ phase, and other hazardous phases.