Abstract: In recent years, with the wide use of computer networks, workflow management system with its purpose of supporting and automating complex business processes have received more and more attention from both research communities and IT industry. Automation of workflow depends on formal description of the business process which computer can recognize to ensure the consistency of data, reliability of process, and cooperation of participants, so process definition is the key to the realization of a WfMS. In this paper, a new business process definition model, XinPai driven Workflow Computation Model which is the theory basis of a Web based workflow management system developed by Software Engineering Institute, Northwest University. This model can describe the dynamic control flow and data flow of the business process at same time, so it’s a kind of extension of general computation model with high expression.