Perfect combination of structural size parameters of the hydroforming billets is essential to obtain even wall-thicknesses of the car-beam.Finite element (FE) analysis on hydroforming car-beam was carried out,and the results were optimized according to multiple quality objectives by the grey system theory.With bending angle,bending radius and hight-difference along the axis direction as variables,orthogonal FE analyses were conducted and the minimum and maximum wall-thicknesses of the billets with different sizes were obtained.Taking the minimum and maximum wall-thicknesses as two references,the correlation coefficient between the data for reference and those for comparison by the grey system theory reduced multi-objectives to a single quality objective,and the average correlation level of every billet facilitated the optimization of size parameters for hydroforming car beam.The trial production showed that the optimization approach satisfied the need of hydroforming car beams.