This paper presents a model-based method to design a software platform, called an application common platform for developments of enterprise information systems. This application common platform(ACP) wraps existing reusable software assets to hide their details from application developers and provide domain level application programming interfaces, so that reusability of software assets and productivity of application improve. In this paper, we present a software architecture which organizes applications, ACP, and software assets and illustrate a development process of ACP. Especially, we show design rules to derive ACP design models from application design models and software assets design models. We also define metrics of reusability and productivity and evaluate the proposed method through real developments of enterprise information systems. As a result, the proposed method reduced 20% of development cost compared to estimation cost.