Abstract: This paper presents a novel pose description corresponding to the structure characteristics of parallel manipulators. Based on the concepts and Grassmann line geometry,the singularity postures of 6-3 Stewart platform are discussed,which leads to an analytic non-singularity zone expression. The configurations of maximum and minimum length of leg in the whole zone are discovered,meanwhile it proved they are true. Based on these results,the authors discover a conservative analytic boundary expression of workspace. The method described here is adaptable to the other cases,so that the kinematics would be solved corresponding to a new pose description of parallel manipulators.