Abstract: Objective In this paper, a variety of welding parameters are used to carry out MAG lap welding experiments on 3 mm thick NM450 steel. Microstructure, weld morphology and fracture behavior of lap joints are analyzed. Methods Weld morphology and microstructure of lap joints are analyzed by optical microscopy and scanning electron microscopy, and fracture behavior is investigated through tensile tests and fracture characterization. Results The results show that lap joints exhibit good formation and heat-affected zone (HAZ) can be divided into coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ), inter-critical HAZ (ICHAZ) and sub-critical HAZ (SCHAZ). Weld metal (WM) mainly consists of ferrite and bainite. Fracture modes are categorized as interface fracture (IF) and HAZ fracture. Fracture mode primarily depends on weld width, and fracture loads of lap joints under IF and HAZ fracture modes are associated with microstructure of WM and crack propagation path, respectively. The average maximum load of lap joints with IF and HAZ fractures are approximately 28.189 kN and 34.561 kN, respectively, with the latter showing a 22.4% improvement in load-bearing capacity. Both fracture surfaces exhibit ductile fracture characteristics, but HAZ fracture surface shows larger and deeper dimples, with more pronounced ductile behavior. Conclusion Weld width is more closely related to the maximum load-bearing capacity of lap joints.