When using GMAW（gas metal arc welding）to weld large gaps with permanent backing T-joint in the box-shaped beam-column structures,due to improper welding structure design,welding parameters and welding techniques and other reasons,a large number of incomplete fusion,solid inclusions,cracks and other defects appear in the root of weld.In this paper,welding tests are conducted with different welding parameters,the welding defects and microstructures are analyzed from the macroscopic metallographic phase and microscopic phase respectively,the causes of the defects are discussed and several proven effective preventive and treated measures are further proposed,which has certain guiding significance for the groove form design and welding technology of components and the processing of the welding backing.