Abstract: Aiming at deficiency of heat affected zone of web’s penetration fillet weld in one project, influence degree of deficiency on properties of weld was determined. Mechanical properties test, microscopic metallography and energy spectrum analysis of the problem steel plate were carried out, and it was found that there were inclusions with different depths. By making simulated welding specimens, the actual product problem was reappeared on the simulated specimens, and destructive test was carried out on the simulated specimen. The result showed that the deficiency of heat affected zone indicated by the flaw detection was caused by inclusion deficiency in the raw material. The deficiency was less harmful and difficult to be completely eliminated, but deficiency of the raw material and multiple repair operations had less effect on mechanical properties of weld. Moreover, the cause of inclusion deficiency of steel plate was analyzed briefly, and the relevant optimization suggestions were provided.