Decision Support Systems (DSS) are computer-based information systems that help decision-makers solve half-structured or non-structured problems by using data and models. They are facing some serious problems in the heterogeneous, autonomic, dynamic and distributed decision support environment. As an advanced technology representing "the third Internet revolution", Grid brings about a lot of innovative ideas and technologies for the development of DSS. As an application of Grid in the field of DSS, this paper puts forward an improved model of Agent Grid Based DSS. A set of design methods of the system is discussed. With the National Economic Mobilization DSS as a practical case, we illustrate how the model works in practice.