Effects of welding heat input on CGHAZ microstructures and mechanical properties of low alloy high strength ship plate steel were investigated by welding thermal simulation method. The result indicated that when welding heart input is smaller,the microst-ructure mainly consisted of lath bainite,granular bainite and some acicular ferrite,the hardness and the impact toughness were higher.The microstructure mainly consisted of lath bainite,granular bainite and lots of ferrites growing from original austenite grain boundary to intracrystalline when welding heat input was 100 kJ/cm. With the increase of welding heat input,the hardness and the impact toughness decreased,which caused mainly by the increasing of granular bainite and coarse grain size. The microstructure mainly consisted of granular bainite,ferrite growing from grain boundary to intracrystalline and grain boundary ferrite when welding heat input was 200 kJ/cm,and the impact toughness decreased further.