This paper presents a novel method for the incremental validation of updates performed on a native XML database system, which can ensure that the updated XML data still conforms to the imposed constraints of the specified schema. On the basis of regular tree grammar theory, we formally describe XML schema languages, and provide representation and storage formats for DTDs in a native XML database. During the process of an update operation, the database first checks whether the expected result satisfies the constraints of its schema. The original XML data is allowed to be modified only when the update operation is accepted. Because only the XML document fragment directly affected by the update is involved in the process of incremental validation, our method leads to a significant improvement to the efficiency of updates for XML data when compared to brute-force revalidation from scratch.