Abstract: The ruler about coarse grain zone impact ductility of low carbon bainite steel welded joint heat affected zone changing with the t8/5 (the cooling time of temperature decrease from 800 ℃ to 500 ℃) was studied, and the effect of heat treatment process after welding on impact ductility of coarse grain zone were researched under the thermal simulation condition when t8/5 was 20 s, then the heat treatment process after welding of the steel was confirmed. The results show that the impacting energy of coarse grain zone in Q550D steel decreased with the increase of t8/5, and the impact ductility of coarse grain zone kept in higher level when t8/5 was no more than 30 s. The tensile and impact properties of base metal changed little when heat treatment after welding was done between 250 ℃ and 600 ℃, coarse grain zone had temper brittleness nearby 350 ℃ and 550 ℃ and kept higher impact ductility nearby 500 ℃. The better heat treatment process of Q550D steel was tempering at 520 ℃ for 1.5 h.