Abstract: This paper compares the X-ray defect quality classifications according to SY/T4109-2013 and SY/T4109-2005 two standards,respectively, and analyzes the rationality of quality grading for the welding defects such as circular defects,strip defect,incomplete fusion,concave,burn through and internal undercut.Combined with the oil and gas pipeline welding practice,some different views on these standards were presented.The author points out that the rating by SY/T4109-2013 standard on the single circular defect is too harsh,and shows the feasibility, with examples, of making comparison of the densograph of porosity and burn through with the parent material.This paper puts forward a proposal for internal undercut defect assessment.