The tube-hydroforming process has undergone extremely rapid development. To ensure a reliable hydroforming process at the design stage, applying robust design methodologies becomes crucial to the success of the resulting process. The reliability of the tube-hydroforming process based on the tube wall thickness thinning ratio is studied in this paper. In order to improve the reliability of the process, the Taguchi method, which is capable of evaluating the effects of process variables on both the mean and variance of process output, is used to determine the optimal forming parameters for minimizing the variation and average value of the thinning ratio. Finite element simulation is used to analyze the virtual experiments according to the experimental arrays. A cross-extrusion hydroformed tube is employed as an example to illustrate the effectiveness of this approach.