In this paper, a systematic synthesis of the optimum arrangements of links and pairs in spatial mechanisms with multiple moving closed circuits is studied with consideration of the degrees of freedom of the mechanisms. Namely, the conditions required to construct the mechanisms with multiple moving closed circuits are formulated. Moreover, mechanisms with multiple moving closed circuits are constructed with some basic links and open chains, obtaining the optimum arrangement of links and pairs. Based on the above analytical results, a type synthesis system is proposed which determines the optimum arrangements of links and pairs. Finally, the types of spatial seven-link mechanisms which satisfy requirements of the above-mentioned basic links and open chains are synthesized to illustrate the method.