SaaS applications have been widely adopted especially by small and medium enterprises. At the same time, the features "multi-tenancy" and "loosely coupled" bring new challenges to enterprises interoperability. On the basis of the layered interoperability model, the paper presents an approach based on interoperability points to implement interoperation between SaaS applications in the service layer. After carrying out the interoperability point matching algorithm, the intermediary Enterprise Service Bus (ESB) performs dynamic selection of interoperability points dictated by Quality of Service (QoS) attributes. In the premise of a comprehensive consideration of the functional and non-functional preferences and constraints, dynamic interoperation between SaaS applications is realized. Finally, this paper shows a case study of applying the interoperability approach.