Abstract: In this paper, multi-layer multi-pass welding ( P1, heat input 3.17 kJ / mm ) and high-power single-pass welding ( P2, heat input 0.94 kJ / mm ) were carried out on 16 mm thick Q500 weathering steel by laser-arc hybrid welding technology. The results show that the width of the heat affected zone ( HAZ ) of the P2 joint ( 1.28 mm ) is reduced by 40 % compared with that of the P2 joint ( 2.1 mm ), and the weld area is reduced by 53 %. The microstructure analysis shows that fine grain boundary ferrite ( GBF ) is formed in the upper part of the weld of P2 joint, and the dendrite spacing of ferrite decreases, but the average size of M-A component increases to 1.8 μm ( 0.7 μm for P1 joint ). The lower part of the weld is dominated by granular bainite ( GB ) + lath bainite ( LB ), which reflects the microstructure stratification dominated by the cooling rate in the thickness direction. In terms of mechanical properties, the tensile strength of P2 joint ( 728.6 MPa ) is higher than that of P1 joint ( 711.2 MPa ), and the strength fluctuation range in the thickness direction is smaller. Low heat input single pass welding significantly improves the comprehensive mechanical properties of welded joints by refining grains and optimizing microstructure uniformity.