Abstract: The accuracy design of a parallel mechanism with three rotational degrees of freedom was studied,which might find its applications in large components pose-adjusting,satellite antenna tracking and high-speed positioning. Firstly,an error model of the mechanism was established by means of the screw theory,in which the compensable and non-compensable errors associated with parts were separated effectively. In the light of interval analysis theory,an appropriate sensitivity index was proposed and utilized for evaluating the influences of non-compensable errors with respect to pose errors of the moving platform. Based upon the sensitivity index,an accuracy synthesis problem subject to accuracy constraints was solved by genetic algorithms,whose objective was to maximize feasible intervals of design variables,ie,the non-compensable errors. Finally,the accuracy design method was validated through the Monte Carlo simulation.