Abstract: Selecting working fluid is an important content of designing single-phase fluid loop system.The chief parameters and criterions for selecting working fluid were considered.The test platform of single-phase fluid loop system was introduced.Based on comparing the chief parameters between the fluid used for Shen Zhou spacecraft and Secool-3 which is one kind of mixture,the experiments at room temperature and low temperature were carried on.The result of the experiments indicates that the single-phase fluid loop system has good performance when Secool-3 mixture is applied.