Abstract: The effects of welding thermal cycle parameters on the microstructure and properties in the heat affected zone (HAZ) of hull structure steel for high heat input welding were studied by thermal simulation technique, optical microscopy and transmission electron microscopy (TEM). It is found that the microstructure of the simulated HAZ is mainly composed of granular bainite, ferrite and pearlite, but it varies markedly with the change of peak temperature and cooling time. The overall level of impact toughness of the simulated HAZ is higher than 200 J, and the impact toughness does not monotonously change with the peak temperature and the cooling time increasing. The amount, size, distribution and shape of M-A constituents in the simulated HAZ are the main influencing factors on the impact toughness.