Modularized and standardized concepts of deeps pace detectors are increasing along with the development of deep space exploration technique. Series connection between different sub-modules of deep space detectors are reasonable and popular, and it is a challenge to propose a reasonable dynamic response index for a single sub-module according to the dynamic response index of the overall series-connected structure. Based on the substructure modal synthesis method and the cantilever beam theory, It proposes a method for assigning design index of dynamic response for sub-modules of series-connected structures. The equation of fundamental frequencies of sub-modules are derived analytically based on three consumptions, and the relation between the dynamics of sub-modules and that of overall series-connected structure are discussed. The proposed dynamics index assignment method shows high accuracy and can provide critical instructions for the design of series-connected modularized deep space detectors.