The steel 42CrMo with thickness of 20 mm used in the planetary frame of heavy trucks, were welded by YLS-20000 fiber laser, and a single-pass penetration weld was achieved using a 20 kW fiber laser. By optimizing the defocus position, a well formed weld bead with no surface defects on both sides was obtained, and the microstructure and mechanical properties of the welding joints were analyzed under hot-rolled and tempered conditions. The experiment results show that melt-through welding of thick plates can be achieved in the case of negative defocusing. Under the condition of Tempered state base material, the microstructure of the weld zone and heat-affected zone were mainly martensite and bainite organization, the average tensile strength of the welded joint is 946 MPa, the average microhardness of the weld and the base material are 509 HV and 247 HV, the microhardness of the weld is significantly higher than the base material, but there is a softening phenomenon in the heat-affected zone. On the other hand, under the condition of hot-rolled state base material, the microstructure of the weld zone and heat-affected zone were mainly bainite organization, the average tensile strength of the welded joint is 781 MPa, the average value of microhardness of the weld and the base material were 506 HV and 219 HV, and the heat-affected zone is not significantly softened. The tensile specimens are fractured at the base material with hot-rolled and tempered state.2023-03-15