Abstract: The influence of welding thermal cycle on microstructures and properties of a high strength low alloy hull steel were studied by Gleeble 3500 simulated tests. The microstructures of samples were observed and the toughness at low temperature was measured. The results show that the effect of t_8/5 on microstructure and properties is not very much under usual heat input range,the microstructures of heat effected coarse grain zone dominated by martensite and bainite,the coarse grain zone is the weak spot in heat affected zone and the low temperature toughness of that may improved by the second welding thermal cycle. There are fine lath substructures and retained austenite in sample after two times of coarse grain zone thermal cycle and the toughness of steel is stable.