<p>This work focuses on the preparation and heat treatment of Q345B low alloy steel produced by laser powder bed fusion (LPBF). An optimized process parameter (<i>Eν</i> = 94.28&#xa0;J/mm³) was proposed for obtaining a high density and hardness of samples. It also evaluates the effect of the tempering temperature on the microstructure and mechanical properties of the LPBF-prepared samples. The results showed that the microstructure of the as-built sample was martensite and acicular ferrite. As the tempering temperature increased, martensite underwent decarburization and transformation into bainite and carbide. As the temperature continued to rise, the mesh-like carbide in the sample disappeared, and the microstructure transformed into ferrite and pearlite. The grain orientation in the as-built sample and the sample tempered at different temperatures were random. As the annealing temperature increased, the content of low-angle grain boundaries (LAGBs) in the sample decreased. When the annealing temperature was 250&#xa0;°C, the tensile strength slightly decreased to 834.5&#xa0;MPa, and the elongation increased to 12.5%. When the annealing temperature reached 850&#xa0;°C, the maximum elongation of the sample reached 14.7%.</p>