Abstract: Effect of welding thermal cycle on microstructure and mechanical properties of copper-bearing age hardening steel were studied. The results showed that there was significant difference on mechanical properties in different region of HAZ after single welding thermal cycle.Coarse-grain region has the poor impact toughness in HAZ because of growth of austenite grain and increasing amount of granular-binaite.Intercritical region is the softened region in HAZ which is due to coarsening and redissolving of precipitate phase and increasing amount of ferrite.Double thermal cycle has great impact toughress on HAZ microstructures and properties.CGHAZ +ICHAZ is region which has the worst mechanic properties after double thermal cycle because of increase of amount and size of granular bainite.