She appeared as a guest judge on the farewell season of the show.
Clarkson also performed at the end of the show. She broke down mid-song and made tears stream down singers Keith Urban and Jennifer Lopez's faces too, reports eonline.com.
She performed her emotional song "Piece by piece" that made everyone become emotional.
The episode also saw Clarkson look back at some of her most memorable moments from her "American Idol" season. (IANS)