The effect of welding heat input ranging from 1.507 KJ/mm to 2.987KJ/mm on low temperature toughness of deposited metal made by self-shielded flux-cored wire has been investigated. The results showed that low temperature toughness decreased with the increase of heat input, and the impact fracture changed from ductile to cleavage fracture. In addition, the chemical compositions demonstrated a slight tendency to decrease with increasing heat input. While, heat input has little effect on inclusions of deposited metal. Higher heat input lead to grains coarsened further, reduced acicular ferrite and granular bainite, and increased polygonal ferrite, which mainly lead to toughness decreased.