Abstract: The effects of trace Zr element on impact toughness of coarse grain heat affected zone(CGHAZ) of 690 MPa grade oil tank steel was investingated with the high heat-input welding cycle using Gleeble3500. The results showed that trace Zr treated steel can significantly improve low temperature impact toughness of base metal, but it could seriously worsen the impact toughness of CGHAZ. The M-A component of trace Zr treated steel coarsed with the heat input increasing,the area percentage of M-A component decreased with the heat input increasing,which resulted in the steel toughness decreasing. The size of inclusions containing Zr are all micron level,and were precipitated in the solidification process,which have litte effect on strenghen and grain refinement,but has a very important influence on impact toughness. The oversize inclusions(1-3μm) could result in the impact toughness decreasing.