The traditional Stewart platform cannot achieve complete dynamic isotropy due to its strict payload re-quirement，while the generalized Stewart platform based on a pair of circular hyperboloid of one sheet gives the ana-lytical form of parameters constraint conditions satisfying local complete dynamic isotropy，and breaks the limit of payload characteristics． In this paper，for the purpose of measuring the modal frequency characteristics of general-ized Stewart platform in large workspaces，a new global dynamic isotropy index is proposed based on the modal a-nalysis． By using this index as the optimal goal and by taking the analytical form of local complete dynamic isotropy as the constraint condition，a structural optimization method，which satisfies the local complete dynamic isotropy and the best global dynamic isotropy in workspace，is proposed． Design example shows that the proposed method provides a novel approach to designing Stewart parallel platform in large workspaces．