Abstract: The effect of Mn content on CGHAZ(coarse grain heat-affected zone) microstructure and properties of large heat input welding in EH36 ship steel were studied by thermal simulation method.The results show that Mn content has a significant effect on the low-temperature toughness of CGHAZ;the best low-temperature toughness in the CGHAZ is obtained when Mn content is 1.2%,but is deteriorated when the Mn content is higher(1.58%) or lower(0.56%).For the EH36 ship steels of large heat input welding,there is also a significant effect of Mn content on the microstructure of CGHAZ,which mainly is GBF(grain boundary ferrite) whose width is about 30 μm when the content of Mn is low(0.56%),and on the other hand,while Mn content is high(1.58%),a large number of M-A constituent is formed in the CGHAZ.The toughness of CGHAZ is determined by both GBF and the horniness phase M-A.