Abstract: The influence of heat input on low temperature toughness of SAW welding seam of marine E36 steel was investigated through metallographic analysis and SEM fracture analysis.The results showed that pro-eutectoid ferrite austenite inter-granular initiates easily columnar crystals is thicker,fracture pattern exhibits cleavage and quasi-cleavage character.The formation of acicular ferrite is promoted in welded seam when heat input reduces and fracture pattern exhibits quasi-cleavage character.Impact toughness of welding seam depends on the proportion of pro-eutectoid ferrite and acicular ferrite.The heat input should be controlled no higher than 32kJ·cm^-1 for the steel thickness equal to 12mm.