Abstract: The realization of bend forming is one of the key link in the design and manufacture of forming die of automatic bending machine. This paper, by discussing the procedure of bend forming, introduces the BSIMU system which employs the CAD technique in the bend forming animation; proposes the related methods of model establishing, the data structure employed in bend forming and its manipulation; describes in detail the complete algorithms and its implementation from preprocessing, intermediate processing, to boundary processing of forming animation. The flow diagram is also given, and some particular cases are discussed. In order to maintain the congruity of geometric relations of workpiece during forming, an algorithm is devised to fetch and correct the geometric information of workpiece. The application scope of the system is also discussed. As a real application of the BSIMU system, a comparatively complicated bend forming case is illustrated.