Abstract: By analyzing the microstructure and HAZ hardness of the welded joint of Q890 D steelat different welding heat input,the effects of welding heat input on the strength of the welded joint and the impact energy at-20 ℃ were investigated. The results show that Q890 DD steel has obvious hardening phenomenon in HAZ. With the welding heat input being increased,the hardness of HAZ increases,while the HAZ hardness of the surface welding bead is much higher than that of the bottom welding bead. This shows that the rear welding bead has an obvious influence on the front weld bead by the heat treatment and the hardened microstructure of HAZ is reduced. The HAZ hardness of the bottom weld bead is lower than that of the weld and base metal,and there is post-weld softening phenomenon. It is necessary to avoid excessive heat input during welding. The increase of coarse bainite in weld results in the low temperature impact energy of weld metal generally lower than that of HAZ. With the decrease of heat input,the tensile strength and impact energy（ including weld,fusion line and HAZ） of welded joints are greatly improved. It shows that the change of welding heat input in a certain range has a great influence on the strength and ductility of the whole welded joint.