Abstract: This article mainly takes the 2000MPa-grade ultra-high-strength steel as the research object. The T Union GM120 high-strength steel welding wire is used as the welding wire. The austenitization temperature of the test steel is determined by the thermal expansion method. By combining the analysis of the microstructure and hardness, the influence law of the cooling time t8/5 on the microstructure and hardness of the coarse-grained heat-affected zone (CGHAZ) in the heat-affected zone of the test steel is studied. Moreover, through theoretical calculations, the cold crack sensitivity of the test steel is analyzed. The results show that when the cooling time t8/5 is between 5 and 30 seconds, the microstructure of the CGHAZ is lath martensite, and the hardness value ranges from 574HV1 to 608HV1. When the cooling time t8/5 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 447HV1 and 513HV1. When the cooling time t8/5 is greater than 300 seconds, ferrite and pearlite appear, and the hardness value is between 311HV1 and 380HV1. When the cooling time t8/5 is greater than 1000 seconds, the martensite structure in the coarse-grained zone basically disappears, and the average hardness value is lower than 300HV1.