MDA provides a nice approach for the automatic construction of software, but pays more attention to the transformation from PIM to PSMs, and neglects that the stage of requirement which affects the accuracy of PIM. In view of such problems, firstly, the paper improves the basic process of MDA, puts forward to the idea that requirements are necessary to be formally described, then the transformation from the requirement model to PIM becomes easier, thus the intention to enhance the accuracy of PIM is achieved. Secondly, by means of the contrast research to three formal languages, the advantages that the B language has in the expression and transformation aspects of the requirement model are found, and the example for how to use the B language to formally describe software requirements is shown. Finally, by combining the framework of model transformation, the graphic descriptions for the B method and the Class Diagram ecore metamodel are given, and further the basic transformation rule from the requirement model described by the B Language to PIM is offered. With the help of tools such as JavaCC, JJTree and ATL plugin, the transformation from the requirement model to PIM is implemented.