Abstract: The influence of welding thermal cycle on microstructures and phase transformation of a high strength low alloy hull steel were studied by Gleeble 3500 simulated tests.Through phase transformation curves analyzing, microstructure observation and hardness testing, the welding continuous cooling transformation curve of the tested steel was constructed.The results show that the microstructures of heat effected coarse grain zone dominated by martensite and bainite under usual heat input range, the tested steel is not sensitive to the alteration of welding thermal cycle.With the heat input increasing, the phase transformation temperature, the microstructures and micro- hardness of heat effected zone of the steel do not change obviously.