As title description, you will find the window pops up the error message as following:
This item cannot be deleted because it is still referenced by other pages.
The delete method:
- Open Site Action-Master Page Gallery, click New -Folder to create a new temp folder.
- Open the site by SharePoint Designer, click left tree menu to open _catalogs\masterpage, and drag the master file into the temp folder.
- Back to IE-Master Page Gallery and press F5 to refresh and then dropdown the temp folder and click [Delete].
- Back SharePoint Designer and press [Delete] button to delete the temp folder and press F5 to refresh.
