In offshore jacket platforms, the structural steels are widely used and usually welded connected. This paper presents the welding of 45# steel plate by LER-3 YAG Pulse Laser. Experiments are carried out to investigate the performance of the welded joint under different process parameters and working conditions. In details, microstructure and macro-morphology in the laser-affected area are analyzed and the micro hardness of the specimens is tested. The results show that: (a) the performance of the welded joint can be improved by the optimization of process parameters, (b) the welding area has the closest grained, the heat affected zone is isometric crystal, (c) the martensite can be reduced by increasing the heat, (d) the grain can be greatly refined by properly increasing the scan speed and decreasing the laser power supply, and that (e) the mean value of the hardness (420HV) in the welded area is obviously higher than that of the substrate (205.12HV) and the highest hardness (492.32HV) appears in the heat affected area.