<p>In this work, AZ31 magnesium (Mg) alloy was fabricated via oscillating laser-arc hybrid additive manufacturing (O-LAHAM), and the effect of various heat treatments on the microstructure and mechanical properties of the sample was systematically investigated. Defect-free Mg alloy samples were successfully fabricated by O-LAHAM, without obvious pores, cracks, or other internal defects. Microstructural characterization reveals that the dominant precipitates are mainly <i>β</i>-Mg₁₇Al₁₂ and <i>η</i>-Al₈Mn₅. All applied heat treatments refined the grains, improved the microstructure homogeneity, and reduced the density of dislocations. Solution treatment promoted the formation of a supersaturated solid solution and suppressed the precipitation of secondary phases, whereas aging treatment facilitated the precipitation of solute atoms and increased both the size and content of precipitates. The T6 (solution + aging) heat treatment achieved a uniform distribution of the fine and spheroidized precipitates, contributing to a distinct enhancement in mechanical properties. After T6 treatment, the yield strength and ultimate tensile strength of the alloy increased by approximately 30&#xa0;MPa, while maintaining a comparable elongation. The main strengthening mechanisms were grain refinement strengthening, solid solution strengthening, and precipitation strengthening. This work clarifies the microstructure–property correlation of heat-treated AZ31 alloy and provides a feasible microstructural regulation strategy for high-performance Mg alloy components fabricated by O-LAHAM.</p>