Multi-DataBase System (MDBS) is a local database of more than one joint. The local database is saved, autonomous, heterogeneous. This paper studies the multi-database middleware actually constructed \"DBMS over DBMS\", which is a multi-database compatible with the general pattern of DBMS solutions. Multi-database middleware is to actually construct \"DBMS over DBMS\", is a multi-database compatible with the general pattern of DBMS solutions. This paper presents designing MSql Server database set in the middleware integration machine. Expression is parsed, the last generation is a dynamic binary tree, and the tree is activated by the calculated expression of results.