Abstract: The 2 000 MPa-level ultra-high strength steel was used as the research object. The Bohler T Union GM 120 welding wire was used. The austenitization temperature of the test steel was determined by the thermal expansion method. By combining the analysis of the microstructure and hardness, the influence law of the cooling time t_8/5 on the microstructure and hardness of coarse grained heat affected zone (CGHAZ) of the test steel was studied. Moreover, through theoretical calculations, the cold crack sensitivity of the test steel was analyzed, which provided fundamental theoretical support and experimental basis for the practical engineering application of ultra-high strength steel. The results show that when the cooling time t_8/5 is between 5 and 30 s, the microstructure of the CGHAZ is lath martensite, and the hardness value ranges from 574 to 608 HV. When the cooling time t_8/5 is between 60 and 150 seconds, the microstructure of the CGHAZ is a mixed structure of lath martensite and bainite, and the hardness value is between 447 and 513 HV. When the cooling time t_8/5 is greater than 300 s, ferrite and pearlite appear, and the hardness value is between 311 and 380 HV. When the cooling time t_8/5 is greater than 1 000 s, the martensite structure in the coarse-grained zone basically disappears, and the average hardness value is lower than 300 HV.