In　the　present　paper,　the　effect　of　stress　on　precipitation　behaviour,　which　decides　the　strength　of　7xxx　alloy,　has　been　investigated　through　a　comparative　analysis　of　the　difference　in　precipitation　behaviour　in　7050　alloy　treated　by　age　forming　and　traditional　aging.　The　results　suggest　that　the　stress　can　enhance　precipitate　nucleation　and　growth　in　age　forming,　which　mainly　attributes　to　two　reasons.　First,　the　increased　diffusion　coefficient　caused　by　applied　stress　field　in　age　forming　induces　precipitates　to　growth.　Second,　in　age　forming,　obvious　dislocation　movement　caused　by　initial　stress　supplied　short　circuit　diffusion　passway　and　nucleation　interface　is　of　benefit　to　the　precipitate　nucleation　and　growth.　Besides,　the　promoting　precipitation　effect　decays　gradually　with　stress　relaxation　during　the　age　forming　process.