Abstract: Due to its excellent characteristics of corrosion resistance, good flexibility and high construction efficiency, polyethylene gas pipelines have gradually replaced steel pipelines in urban gas transportation and become the main transportation method.Installation of polyethylene pipes mainly adopts electrofusion and hot fusion welding.The main welding defects are over welding, cold welding, unremoved oxide scale, dislocation of resistance wire, holes, and so on.Effect of different welding defects on strength of polyethylene welded joints and influence mechanism was studied.Welded joints with artificial defects were analyzed and tested by tensile, flattening, SEM and EDS test methods.The results of tensile and flattening mechanical properties showed that cold welding, over welding, unremoved oxide scale, and dislocation of resistance wire were main defects affecting strength of welded joints of polyethylene pipes, and unremoved oxide scale had the greatest impact on strength of welded joints, which was 0.4% of fracture stress of normal welded joints.SEM and EDS test results showed that molecular chain of pipe containing oxide skin was prone to crosslinking, chain scission, and poor fluidity.Diffusion of macromolecular chains of polyethylene base metal was hindered by hard plaques by thermal and oxygen aging and mechanical scratches.Under action of ultraviolet rays, light, heat, and so on, oxygen reacts with the molecular chain on surface of polyethylene to form a large number of hydroxyl radical groups, which made polyethylene molecular chain undergo thermo-oxidative aging and degradation, and greatly reduced strength of polyethylene joint.