To overcome the limitations of both approaches classical and formal for the development of complex software, we proposed a hybrid approach combining the formal approach (Event-B) and the classical approach (UML/OCL). Upstream phases of our approach include: Rewriting the requirements document, Refinement strategy, Abstract specification and Horizontal refinement. We have shown the feasibility of our approach on a case study: An Electronic Hotel Key System (SCEH). The problem of transition from the formal (Event-B) to the semi-formal (UML/OCL) is processed through our extension to OCL (EM-OCL).