By using YLS-10000 fiber lasers and ERNiMo-8 filler wire, the ultra-narrow-gap laser welding of 9Ni steel for liquid container of liquefied natural gas (LNG) vessels is conducted and the microstructure and property of the welded joint are studied. The results show that, the ultra-narrow-gap welded joint with good fusion sidewalls can be obtained by optimizing weld parameters. The microstructure of weld is austenite, and crystallization morphology on two sides is columnar crystal, while that at center is equiaxed crystal. The heat affected zone (HAZ) is clearly divided into coarse grain zone and fine grain zone, and both of their microstructures are martensite and a small amount of residual austenite, where the average hardness is about 340 HV, much higher than that of the base metal (230 HV) and that of the weld (200 HV). Fractures occur on welds of tensile test specimens, and the tensile strength of the joint is slightly lower than that of the base metal. The low-temperature impact energy of the joint increases with its distance from the weld center increase, with an average value of above 70 J. The fracture morphology is always tiny dimple and the fracture of the joint is ductile.