Abstract: Coupling the system dynamics with multi-objective planning (SD-MOP), a relevant model was developed taking account of their respective advantages and disadvantages. Then, the Changshu city was taken as an instance to which an optimization analysis was made systematically and dynamically. In the analysis the five sub-systems including economy, population, resource, space and ecology were taken as cores to give a cause-effect feedback graph for the city size. With the city's population and per capita living space taken as reference objects, the conformity and sensibility tests were carried out and verified that the SD-MOP model is stable with small error. Furthermore, taking the regulation coefficient of technologies, rate of land exploitation, improvement rate of ecological condition and increment of greening coverage rate as control parameters, the optimization analysis was made for the Changshu city size.