To enhance the thermal efficiency and reduce surface defects in magnesium alloy welding, a power-modulated oscillation laser welding technique was proposed. Butt welding was performed on 5 mm thick AZ31B magnesium alloy, and the surface formation, microstructure, and mechanical properties of the weld were analyzed. Additionally, numerical simulation and simulation analysis of the welding temperature field were conducted. The results indicate that compared to the effects of conventional laser welding, welding defects decrease significantly, weld width increases, and energy coupling efficiency is improved with power-modulated oscillation laser welding. The grains in the center of the weld are refined, and the formation of β-strengthening phases increase, leading to enhanced mechanical properties, with the ultimate tensile strength and elongation reaching 96.1% and 58.7% of the base material, respectively. The temperature at the center of the weld is reduced by approximately 500 K, resulting in a more uniform heat distribution and a decreased temperature gradient in the melt pool. These findings have significant practical implications for improving the performance of magnesium alloy joints, lightweight transportation, and energy conservation and emission reduction.