Abstract: The microstructure and properties of coarse grain in heat affected zone (CGHAZ) of 12 CrNi5 MoV forged steel were studied by using welding thermal simulation technology. The result show that after one welding thermal cycle, the tensile strength of CGHAZ significantly increased compared with that of the original sample, and the impact toughness significantly decreased. The mechanical change of CGHAZ was attributed to the increasing of twin martensite and retained austenite contents in the CGHAZ which resulted from the increasing of cooling rate.