UML is used to describe a variety of models by MDA .UML is an actual standard for modeling languages. Because of the lack of constraints over the realization of relational database models in UML class diagram design, the outcome of the transformation from the UML class diagrams to the realization of relational database models is not exclusive. It can not sufficiently achieve the designer's intentions on the database and it is inconvenient in doubledirectional transformation from PIM to relational PSMs in MDA. To solve the problem mentioned above, an approach is proposed to enhance the relationship and the constraints of data in class diagrams which can extend UML by adding stereotypes and OCL constraints to build exclusive relational PSMs from PIM. Finally, a method based on QVT is provided for transferring the extended UML class diagrams to relational database models. In several examples the transformation rules are defined for association, inheritance, aggregation and combination of the UML class diagrams etc. It can get the exclusive outcome of the transformation from the UML class diagrams to the relational database models using this method.