Errors model of 6-PTRT parallel robot is proposed by setting up the relation of pose errors and structural parameters based on the closed-vectors differential. Errors sensitivity index are defined by using of singular value of the matrix and are given to physical meanings. Errors synthetical sensitivity, direction sensitivity and maximum sensitivity are quantitatively analyzed through simulation, which prove that errors sensitivity of some feature in the working space can be of evaluation scale for optimum structural design. The errors model set up can lay a solid foundation for structure design and errors compensation of the robots.