Thank you for your reply and sorry for being my lazyness, bcz I still use drop 9 for current project.
"Copy Local = False" already set and it works without problem in drop 9.0.
The difference is drop 9 uses AggregateCatalog and adds deploymentCatalog after xap downloading completed as such;
however, drop 10 add downloadcatalog into it's own colelction without adding into AggegateCatalog as such;
foreach (ModuleInfo moduleInfo in moduleInfos)
so all Parts of downloaded xap can't be seen in AggregateCatalog, though existing in deplocatalog.
Should I take additional acton to add deploymentCatalog to AggregateCatalog ?