Abstract: In this paper, thermal simulation technology, microscope analysis and fracture surface topography analysis techniques are used to study the influence of heat input on the impact toughness and embrittlement mechanism of coarse grain zone in butt weld of QT900 coiled tubing.The result shows that fine needle lathe bainite can be obtained under 5 kJ/cm heat input, it improves impact energy and provides good impact toughness to coarse grain zone; with the heat input increaseed from 5 kJ/cm to 10 kJ/cm, grain size increases in coarse grain zone, bainitic ferrite laths become widen, and the grain boundary allotriomorphs will nucleate and grow along the original austenite grain boundaries, which decrease toughness in coarse grain zone.