Abstract: Based on the internal corrosion of the square elbow of one typical pipe fitting from a gas field,the flow field of pipe fitting was analyzed by the method of CFD software FLUENT,and the morphology and composition of corrosion product was analyzed through EDS and XRD.At the same time,the study was carried on the metallographic analysis of pipeline weld heat affected zones,and the chemical composition of piping material was analyzed.The comprehensive analysis results showed that:the direct corrosion reason was carbon dioxide corrosion caused by the water sedimentation and erosion corrosion caused by the high velocity;the indirect reason was that the weld defect accelerated the corrosion process.