<p>Machining accuracy is a crucial indicator for five-axis ultra-precision machine tools. While test pieces are commonly used for performance evaluation in machining processes, existing designs are primarily suited for conventional or precision-grade machine tools, making them inadequate for capturing the linkage geometric errors and dynamic errors in five-axis ultra-precision machine tools. To address this gap, a novel eccentric sphere test piece was proposed in this paper, along with a comprehensive methodology encompassing design, machining, measurement, error analysis and compensation. The practical utility of this approach was demonstrated through a 57.2% reduction in sphericity error achieved via a compensation strategy derived from measurement results and error model. This study contributed a specialized test piece and a quantitative machining-compensation method, which together offered a practical tool for enhancing machining accuracy in five-axis ultra-precision machine tools.</p>