Abstract: To investigate the damage situation of welding joints of ethylene cracking furnace tubes after long-term service under high temperature, the microstructure, high-temperature endurance property and hardness of welding joints of Cr35Ni45Nb alloy after a six-year service was analyzed. The results show that:after a high temperature and long term service, the primary eutectic carbides in base metal and welding seam of welding joints of Cr35Ni45Nb alloy coarsened and formed a continuous network structure, and the secondary carbides agglomerated together to blocky shape; the carbides of M₇C₃ and NbC transformed into M₂₃C₆ and G phase respectively; the high-temperature endurance property of welding joints decreased and the hardness increased; the area around the fusion line was the weakest zone of the entire welding joints; the damage of the heat affection zone of welding seam of ethylene cracking furnace tubes was comparatively serious after a 6-year service, and the furnace tubes should not be used any more.