Abstract: By using welding thermal simulation technology,coarse grained region microstructure and properties of high heat input welding DH36 steel were studied.The results show that low C,low Mn and low carbon equivalent with Ti treatment alloy design significantly improve the toughness of coarse grained region.The microstructures of coarse grained region change from thin lath M-A island to massive M-A island,and the amount of pro-eutectoid ferrite increases with the increasing of t_8/5.The island with size larger than 1 μm shows significantly effect on the toughness of coarse grained region.Decreasing the Mn content can promote the formation pro-eutectoid ferrite and inhibit the formation of M-A hard phase at the same time,but excessive coarsening pro-eutectoid ferrite will decrease the low temperature toughness of coarse-grain HAZ obviously.The interaction of the M-A islands and pro-eutectoid ferrite affect the high heat input welding mechanical properties of DH36 steels.