Abstract: Crack defect is often found in tube filled weld on power boiler set box,which usually causes pipe leakage.In order to avoid this defect,numerical simulation of the tube filled weld was carried out.In this paper,causes of crack were discussed and the influence of the incomplete fusion defection was studied.Then the validity of security analysis method for tube filled weld was proved,and the scientific basis was provided for the overhaul of the power station.