  We study the relation between entanglement and quantum phase transition (QPT)
from a new perspective. Motivated by one's intuition: QPT is characterized by
the change of the ground-state structure, while entangled states belong to
different classes have different structures, we conjecture that QPT occurs as
the class of ground-state entanglement changes and prove it in XX model.
Despite the classification of multipartite entanglement is yet unresolved, we
proposed a new method to judge whether two many-body states belong to the same
entanglement class.
