The design and process of diaphragms for a welded bellow was studied in this paper, which is a flexible component in a piping system or vacuum equipment. Due to a lot of interactive effects on each procedure in the manufacturing for a welded bellow, furthermore the commercial FEM software ANSYS was used to construct a 3-dimension model for the displacement and modal analysis of bellows in which extensions and natural frequencies as well as mode shapes could be observed. The mechanical feature should be considered at the practical applications, i.e., to avoid the occurrence of resonant phenomena corresponding to pump operations and fluid delivery in a piping system. For the purpose of modeling validation, the axial tensile testing machine implemented the extension experiment for welded bellows. According to the results of simulation and experiment, the difference among them could be easily detected for the feature investigation for a design-oriented systemic analysis on the development of welded bellows.