Abstract: Effect of different heat inputs on microstructure and low temperature toughness of 550 Mpa offshore steel welding joints was studied. The experimental results indicate when the heat input is 15 kJ/cm or 50 kJ/cm,the welding metal experience the effect of heat treatment,and the microstructures are acicular ferrite and fine granular dispersed carbides,especially when the heat input is 50 kJ/cm. In the impact test at -60℃,when the heat input is 50 kJ/cm,the impact energy is the highest, which is 110 J and 2 to 4 times of that obtained under other heat inputs. The microstructure in the fusion line and 2 mm from fusion line are lath bainite and granular bainite. When the heat input is small,a spot of granular bainite disrupt the direction of lath bainite,which is good for low temperature toughness. But the granular bainite increases and gets coarse with the heat input increasing,which leads to bad low temperature toughness. The impact value in these zones decreases with the heat input increasing.