Experiments of high power laser welding of Y gap ship plate have been performed using fast axial flow 10 kW CO2 laser, five-axial computer numerically controlled (CNC) table, St370-2 ship mild-steel plates and SG filling wire. Morphology and microstructure of laser welding seam have been widely examined by optical microscope (OM), scanning electron microscope (SEM) and X-ray diffractometer (XRD). It is shown that no crack full penetration welding in 12 mm thickness ship plate can be achieved at laser power of 8～10 kW and welding speed of 1 m/min. Comparing with submerged-arc welding (SAW), laser welding has a narrow seam width of 5 mm and a small heat-affected zone of 0.3 mm. Microstructure of fine martensite and a small amount residual austenite has been observed in the laser seam, which has a good synthesis mechanical obdurability. Mechanical tests of hardness of HV350, tensile strength of 536 N/mm2 and 180° cold bend without fracture have been measured.