This paper presents an approach to provide a Constraint Service for XML-based Data Integration Systems. Constraints from local data sources and global constraints are expressed in a uniform constraint model based on Active XQuery and are stored in Constraint Repository. We introduce the concept of Constraint Wrapper and translate local constraints from data sources automatically into the uniform constraint model through Constraint Wrapper with the help of schema mapping information. The Constraint Service can be published as web service and be invoked by different Data Integration Systems. Through the Constraint Service, queries can be optimized, and global updates can be checked for validity and integrity. We establish the DIS-CS system to facilitate both the architecture and the implementation. We carry out experiments of Semantic Query Optimization using Constraints. The results show that the performance of the queries is greatly improved.