Abstract: Laser welding of 6mm-thick CLAM is conducted with different welding parameters by 4 kW Nd:YAG.Microstructure and hardness of welded joint are analyzed with pre-and post tempering respectively.The results show that the microstructure of welded joints are mainly coarse lath martensite and hardness of welded joint is increased as welding speed rises before tempering.After tempering,microstructure of welded joint is tempered lath martensite.A large amount of carbide particles are observed in original austenite grain boundary and martensite lath by SEM,which lead to a slight increase in the hardness of weld zone to base metal relatively,HAZ is not softened.