Abstract: Niobium is a strong carbonitride-forming element. The evolution of Nb-rich carbonitride in austenitic steels during welding has an important effect on the ductility of the heat-affected zone (HAZ). The new austenitic heat-resistant steel of 22Cr15Ni3.5CuNbN, a candidate material for ultra-super critical boiler superheater and reheater serviced at 620-650 ℃, contains 0.5% Nb, which will significantly affect the steel's weldability; therefore, it is necessary to study the microstructure and properties of the weld HAZ of the steel and provide a reference for the further applications of this new material. Because of the narrow weld HAZ of this material, the extended HAZ structure of 22Cr15Ni3.5CuNbN austenitic steel at different peak temperatures from 1150 ℃ to 1300 ℃ was obtained by Gleeble thermal physical simulation method in this study, aiming to simulate the thermal cycling process of the welding process, and impact performance tests were carried out. The results show that a certain amount of Nb-rich composite carbonitrides is present in the base metal of the experimental steel, which effectively pins the grain boundaries and entangl with a large number of dislocations. The Nb-rich composite carbonitride underwent a complex process of dissolution and re-precipitation during the simulated welding process. When the peak temperature was at 1150 ℃, only small particles of Nb-rich carbonitrides were dissolved, while when the peak temperature reached 1300 ℃, the Nb-rich composite carbonitride underwent dissolution and re-precipitation, showing a "mesh" structure, and its overall size increased. The evolution of Nb-rich composite carbonitrides led to changes in the impact energy of this steel. The impact toughness of the experimental steel subjected to welding thermal cycling condition was higher than that of the base metal. With the increase in the peak temperature, the impact toughness first increased and then decreased. The impact toughness of the steel reached the highest when the peak temperature was at 1150 ℃.