Abstract: The installation of a sphere-wrist at the end of the manipulator is acommon characteristic of many robots. The sphere-wrist can be regarded as an important independent structure of a manipulator. Static errors of sphere-wrist will affect the pose precision of the end effector. Taking sphere-wrist as study object, a comprehensive criterion to evaluate its precision was presented on the basis of the analysis of static errors. Using this criterion, an optimization model was constructed to rebuild the model of the sphere-wrist that had static errors, and hence a new kinematic model of the sphere-wrist was constructed. When this new model was used in the kinematic and dynamic computing, the pose error of the sphere-wrist could be reduced evidently. This optimization method will be more beneficial to a sphere-wrist with big static errors. By the partial optimization to the robot with a sphere-wrist, the kinematic accuracy of the end effector of the whole robot can be increased to some extent.