Abstract: In order to investigate the effect of line energy on the forming and performance of laser-arc composite welding of AH36 high-strength steel, a butt welding of 8 mm thick AH36 high-strength steel was performed by using the laser-metal active gas (MAG) composite welding method under different process parameters, and the laser energy, the arc energy, and the energy density distribution were analyzed respectively. The results show that under the process parameters of laser energy of 10 kW, wire feed speed of 8 m/min, and out-of-focus amount of −2 mm, a defect-free weld with excellent performance was obtained, with a maximum microhardness of 380 HV at the centre of the weld, a tensile strength of 578.3 MPa, and a maximum work of impact of 62.2 J. Within a certain range, the laser energy mainly affects the formation of the backside of the weld, and the number and volume of the backside hump decreases by increasing the laser energy. The arc energy mainly affects the frontal shape of the weld. The microstructure of the composite action zone in the centre of the weld consists mainly of acicular ferrite (AF) and polygonal ferrite (PF), while the laser zone is mainly composed of lath martensite (LM) and bainite (B). Weld centre laser area microhardness is higher than the composite area, increase the line energy, composite area weld centre microhardness from 350 HV to 300 HV. The tensile fracture positions of the welded joints obtained at different line energies are all located in the base material, and the tensile strength reaches more than 575 MPa. Increasing the line energy improves the impact toughness of welded joints up to a maximum impact work of 85.5 J. This study provides a basis for the application of laser-arc composite welding of AH36 steel in shipbuilding.