Abstract: The quantitative relationships of static stiffness, lightweight factor and modal parameters of aluminum-alloy body platform are derived according to modal theory, to provide a guidance for the lightweight design and the performance objective setting of aluminum-alloy body platform in early concept-design stage. Then based on the finite element model, the former 50 orders of modal parameters are extracted and the approximate solutions of bending stiffness, torsion stiffness and lightweight factor of aluminum-alloy body platform are calculated, which are very close to that of finite element analysis with their relative errors being only 4.32%, 1.85% and 1.78% respectively. So, it can be found that the static compliance of aluminum alloy body platform can be approached by the sum of the modal compliance of each order, and the vibration mode with the most contribution to bending (torsional) stiffness is right the first order bending (torsional) mode, being a conclusion as an important way of modal identification. Finally, the bending and torsional stiffnesses and lightweight factor obtained by finite element analysis and modal theory algorithm are compared with that by test and it is quite evident that the error of modal theory algorithm is smaller than that of finite element analysis. They are as small as 1.85%, 1.82% and 1.89% for bending stiffness, torsional stiffness and lightweight factor respectively

Key words: aluminum alloy body platform, stiffness, modal, lightweight factor