Abstract: Locking mechanism plays a key role in transaction processing of DaaS based on XML databases.Traditional research on locking mechanism has many limitations,such as poor concurrency ability,and do not support main query language such as XQuery.This paper proposes a multi-granular locking mechanism,it creates semantic locks on the nodes of dynamic pattern tree view for every database node,and manages the transaction coordination of different databases with the global pattern tree view of the main database node.A discussion about the differences between the locking mechanisms of this paper and current are proposed in the last of the paper.