A method for automatically replacing reels (10', 10") in an unwinding assembly (30) capable of carrying out the handling operations of the reels (10', 10"), as well as preparing and joining the tail (12't) of the web (12') of the finished reel (10') with the head (12"h) of the web (12") of the new reel (10") in a simple, efficient way, reducing the risk of malfunctions and maintaining the correct orientation of the new web (12") as that of the finished web (12').