Abstract: For performance evaluation of zero-g simulation device of space manipulator based on suspension system, an "unloading ratio" index is proposed, and methods for improving simulation reliability and accuracy are investigated. Based on the small deformation assumption and principle of linear superposition, the subsidiary stress and bending-torsional deflections of links are analyzed and errors of the 2-dimensional follow-up system and tension force errors of slings are taken into consideration. The linear relationship between the subsidiary stress and tension force errors of slings is set up, and a definition of "influence degree" is proposed which maps the tension force errors of slings to the subsidiary stress of links. Then the "unloading ratio" of a single sling and the "total unloading ratio" index of the zero-g simulation device are defined. Finally, methods for calculating the "unloading ratio" index and relationships among indexes are investigated.