By integrating system thinking and social psychology, this paper presents an Activity System Theory (AST)1 approach to the platform design of e-service systems in general, and e-healthcare systems in specific. In the first part, some important principles of AST and a sustainable model of Healthcare (IMIS) is presented to demonstrate how to construct a comprehensive platform for various complex e-service systems based on the sustainable model of AST. Our research focused on the complex e-healthcare system in Sweden, and the results showed that the model of AST can provide the designers of e-service system with a comprehensive and sustainable platform for designing various kinds of e-service systems.