Abstract: The relationship between HAZ structure and properties of HQ70 low-carbon low-alloy Q and T steel under multiple weld thermal cycle simulating conditions was studied.It was discovered that structural heredity phenomenon took place in the non-equilibrium structure of overheated zone of HQ70 steel after another quick heating.The temperature range of structural heredity was found to be approximately 750~1050℃.Stuctural heredity worsened the embrittlement of HAZ overheated zone.The causes of embrittlement were analyzed and discussed,and the measures for improving the effect of structural heredity on toughness studied.