Abstract: By simulating the welding thermal cycle,the continuous cooling transformation diagram of heat-affected zone (HAZ) of ZG0Cr13Ni5Mo martensitic stainless steel was established.The diagram indicates that there are no other transformation but austenite→martensite in HAZ,and the cooling rate has no obvious effect on the temperature Ms of martensite initial transformation.The hardness of simulated HAZ decreases slightly as the cooling rate decreases.By using Gleeble thermal simulation technique the microstructures of HAZ at various welding thermal cycles were simulated,and the hardness,impact toughness and microstructures were investigated.The results show that the hardness of simulated HAZ is much higher than base metal's hardness,and the impact toughness of simulated HAZ is much lower than the base metal's.In the single thermal cycle simulating the root pass welding,the cooling rate has no effect on hardness and impact toughness of simulated HAZ.In multi-thermal cycles simulating the multi-pass welding,the heat treatment between passes and layers is not significant.Postweld heat treatment at 590℃ is an essential way to improve the impact toughness of HAZ greatly.