Abstract: The microstructure and mechanical properties of coarse grain heat-affected zone(CGHAZ) of 610 MPa high strengthen steel which were treated under series high weld heat input by MMS-200 thermal simulation machine were studied.The results show that the test steel has good ability to resist thermal shock of 80-100 kJ/cm welding heat input,even after 120 kJ/cm welding heat input,the low-temperature toughness of heat-affected zone still keeps high level.With the increasing of welding heat input,the microstructure of CGHAZ gradually becomes more coarse and the toughness gradually worse;there are many fine and dispersed second-phase particle of TiN in test steel,which can prevent growth of primitive austenite,promote phase transformation of ferrite as nucleus and make the steel to resist thermal shock of high welding heat input.