The syntactic behavior of idioms is a matter of significant debate, in that it is closely related with conflict between syntactic status and lexical status of idioms. This writing reviewed research on the syntactic behavior of idioms, and then examined the status conflict, which needs to be explained in terms of semantic motivation. The metaphoric interpretation of the idioms`` constituents is suggested as the effective measure which can explain why some idioms can be syntactically changed and still retain their figurative meanings, while other idioms cannot be syntactically altered without losing their figurative meanings. The syntactic changes of idioms are motivated people``s intuitions about different knowledge domains to illustrate the meanings of idioms, and source-to-target domain mappings of the metaphors.