With the rapid development of network technology, current workflow system has become service-based. But the services in current service-based workflow system are either totally public or hidden. The paper presents a service-oriented workflow model which is composed of two types of workflow service: Private and Public. The private service is the full definition of a workflow service as common. The public one is simplified from the private service according to a principle — Interface Equivalence. After taking an overview of the service-oriented workflow model, it provides several definitions which are required in Interface Equivalence. Given the property of Interface Equivalence, it proposes an algorithm to simplified public service from private service based on the Interface Equivalence.