Healthcare information system is a very complex system and has to be approached from systematic perspectives. This paper presents an Activity System Theory (ATS) approach by integrating system thinking and social psychology. First part of the paper, the activity system theory is presented, especially a recursive model of human activity system is introduced. A project ‘Integrated Mobile Information System for Diabetic Healthcare (IMIS)’ is then used to demonstrate a practical application of the Activity System Theory especially in constructing healthcare information system. Our conclusion is that the activity system model can provide the service system designers with a comprehensive and integrated framework for designing healthcare information system in specific, and for designing various kinds of service systems in general.