Abstract: Austenitization temperature of a 1300 MPa grade high strength low alloy steel was measured by the Formastor-FⅡ thermal expansion measurement, and the effect of cooling time from 800 to 500 ℃(t_8/5) on the microstructure transformation and hardness change of the coarse grained heat affected zone was investigated by using Formastor-FⅡ, optical microscope, vickers hardness tester. The experimental results indicated that the microstructure of coarse grained heat affected zone (CGHAZ) was composed of lath martensite with hardness ranging from 438 to 454 HV5, when the t_8/5 was 3 ~ 60 s. Along with the extension of cooling time, bainite began to form. When t_8/5 was 150 s, the microstructure was mixtures of bainite and martensite with an average hardness of 413 HV5. The microstructure was mixtures of lath bainite and granular bainite with hardness ranging from 341 to 381 HV5 as t_8/5 was 300 ~ 600 s. When t_8/5 was longer than 600 s, the microstructure mainly consisted of granular bainite with hardness ranging from 269 to 322 HV5. The cold cracking sensitivity evaluation results show that the carbon equivalent C_E (IIW) and C_EN of the test steel are both greater than 0.5%, which has a certain tendency of cold cracking. Therefore, it is necessary to preheat welding, heat treatment after welding or heat preservation and slow cooling measures to avoid the formation of welding cold cracking.