Abstract: To investigate the essential reasons for the effect of heat input on the low-temperature impact toughness of the CGHAZ of Ultra-strength steel, the CGHAZ simulation specimens were prepared by using the thermal simulation. Combined with the instrumented impact test and the quantitative characterization methods of microstructure by OM and EBSD, the influence of microstructure and grain size on impact toughness was analyzed. The results show that with the increase of heat input, the content of microstructure changes significantly in the range of 27 - 53 kJ/cm; the PAG grows rapidly in the range of 30 - 40 kJ/cm; the frequency of HAGB increases rapidly and the diameter of EG decreases rapidly in the range of 50 - 60 kJ/cm. When the heat input is 10 kJ/cm, brittle fracture occurs; when the heat input is 30 and 100 kJ/cm, ductile fracture occurs; when the heat input is 40 and 50 kJ/cm, it is in the transition state of ductile-brittle fracture. When the heat input is in the range of 10 - 30 kJ/cm and 60 - 100 kJ/cm, the low-temperature impact toughness of CGHAZ is mainly affected by the content of microstructure; when the heat input is in the range of 30 - 60 kJ/cm, the main influencing factors of the low-temperature impact toughness of CGHAZ change from the diameter of PAG to the frequency of HAGB and the diameter of EG.