Abstract: The microstructure and property of the coarse grained heat affected zone( CGHAZ) of 960QT steel was studied under t_8/5 by using thermal simulation technology, and the continuous cooling transformation curve of the heat affected zone(SH-CCT diagram)was acquired. The results show that, the microstructure of the CGHAZ consists of lath martensite and the hardness values change little as t_8/5  is 3 - 6 s. When t_8/5 = 10 s, lath bainite appears. With increase of t_8/5 , the lath martensite reduces and lath bainite increases, and the microhardness of CGHAZ gradually reduct. When t_8/5 reaches 150 s, granular bainite in structure is found the microhardness is further reduced. As t_8/5 > 600 s, the CGHAZ mainly consists of granular bainite. Based on the measured heat affected zone 5, the relationship between tg/5 values and heat input E of steel 960QT is established. As t_8/5 is 30 s, the microhardness of CGHAZ (325 HV1) is equal to the base metal(330 HV1). So the heat input should be controlled to ensure that t_8/5 is less than 30 s. Under non-preheating conditions, the heat input should be less than 1.4 kj/mm.