Abstract: The structure and properties of HAZ of X80 pipeline steel were studied on the basis of physical simulation technology and microstructure analysis method.The causes of HAZ toughness worsening were analyzed.The worst weakened zone in weld was indicated.It is show that the critical coarse grain heat affected zone is the local brittle zone of X80 HAZ where the toughness is relatively low.When the peak temperature of the second thermal cycle is in the temperature range of two-phase zone (α＋γ),the content size and hardness of M-A constituents are relatively high. Those are the main factors resulting in the local brittleness.