Abstract: The mierostructure and toughness in the welding heat affected zone (HAZ) of boron containing high-strength low-alloy (HSLA) steel plates were investigated by means of thermal simulation with a Gleeble thermal simulator. The results show that the toughness in the overheated zone decreases significantly with the increase of cooling time t_8/3. When t_8/3 below 67 s, the impact toughness in the overheated zone is higher, the structure is lath martensite or lath martensite + bainite, and the grain is finer. Since the impact toughness in the HAZ and weld joint of 800 MPa grade low carbon bainitic steel is higher, the welding process which are 0.96-2.11 kJ·mm^-1 heat input, 150-200℃ interbead temperature and heat treatment after welding has a high adaptability to the low carbon bainite steel.