As a test piece for detecting the dynamic accuracy of five-axis milling machine tools, S-shape test piece has been applied to ISO 10791-7 groups to be an international standard specimen. The profile of S-shape test piece is constituted by two ruled surfaces. However, the exactly parameterized method of ruled surfaces has not been included in the S-shape piece draft,which may cause the non-unique expression of S-shape test piece. Different definition methods of ruled surfaces may have important effects on the detecting characteristics of S-shape test piece. In order to address the above issue,two typical parametric definition methods of ruled surfaces are provided in this paper, which are iso-parametric method and arc length method. Furthermore,iso-parametric method is chosen as the profile definition method of ruled surface of S-shape test piece according to the analysis of the twist angle and the theoretical machining errors. The research improves the definition theoretically, which may be beneficial to the promotion of S-shape test piece.