Abstract: The effect of different welding parameters on the weld of KMN steel plate were studied through the orthogonal test of autogenous laser welding. The results showed that the laser spot diameter had the dominant effect on the weld penetration degree and the weld surface quality in the selected parameter range. The preliminary optimized welding parameters through orthogonal test were spot diameter of 0. 8 mm,welding speed of 24 mm/s and laser power of 4. 2 kW. Two sets of welding parameters with different energy density were selected to investigate the change of macroscopic morphology,microstructure and microhardness with welding parameters. It was found that the higher laser energy density gave rise to the deeper and wider weld,the coarser lath martensite and the lower hardness in the corresponding regions. Due to the differences between single-plate autogenous laser welding and butt autogenous laser welding,the practical welding parameters for butt-weld should be obtained by an appropriate adjustment. Laser butt welding was carried out with the adjusted welding parameters,and the mechanical properties between the obtained joint and TIG welded joint were compared. It was found that the laser welded joint had a higher tensile strength and elongation,but a lower impact toughness.