Workflow management systems (WfMSs) become the basic technology for organisations to build their Information Systems. To understand the business processes already implemented in the existing software systems and then build the workflow oriented Information System is a time-consuming and error prone process. This paper proposes a unified software reengineering approach from a business process perspective. A workflow extraction method is developed to elicit the business processes from existing systems. A precondition-based workflow model is designed for this purpose, which is an activity-centred method for program analysis. The calculation of the activity’s whole condition provides powerful analysis techniques to verify the correctness of the recovered workflow model. Through the proposed approach, the workflow procedures can be recovered from the existing system and verified by the precondition analysis.