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Thick plate multi-pass multi-layer narrow gap laser non-autogenous welding technology was very important in the fields of shipbuilding, pipelines, nuclear, submarines and so on.However, the weld was hard to clean after welding because of the narrower groove size that would decrease the joint strength by traditional method.In the present paper, we describe the application of laser cleaning technology in thick plate narrow gap multi-pass multi-layer laser non-autogenous welding process.An orthogonal test was designed to optimize technical parameters, including laser power, defocusing amount and cleaning speed.The comparison between weld cross sections and the amount of oxide on weld surface proved the laser cleaning method do play an important role in cleaning the weld.The tensile strength improved obviously because of having little welding defects in the weld under this condition.Combining this cleaning technology and the laser non-autogenous welding system comprised the laser autogenous welding,laser wire filling welding and the laser arc hybrid welding, 40mm thick plate was welded well using 4kW fibre laser.