This invention discloses an expading method for user terminals of a subset difference/hierarchical subset difference system including: setting up an expanded logic ciphered key tree with an expanded user terminal as the leaf node, merging the expanded tree with the original tree before expansion to form expanded ciphered key trees, distributing independent key labels for other sub-trees and nodesto set up a key system with unchanged key label and system of the original user terminal before expansion, setting up pre-distributed keys of the expanded user terminals with unchanged pre-distrubutedkeys of the user terminals before expansion, encrypting M and setting up broadcast information transmitted in channels when transmiting tip to any subset of the user terminal, which analyzes and deciphers the broadcast information to get cleartext of M.