The invention discloses a body-in-white light weight optimization design method, and particularly relates to the field of body-in-white light weight, and the optimization design method comprises the following steps: S1, determining a single-degree-of-freedom system model, and determining a displacement curve of a system and a structure during vibration based on a structural dynamics generalized coordinate method, S2, building a system motion equation, employing a rigidity method of a direct balance method, and building a calculation model; s3, comparing equation data with the body-in-white structure, whereini inertia force FI represents braking performance data of the body-in-white structure, and elastic force FS represents bending, turning and torsion performance data of the body-in-whitestructure. By changing materials, optimizing the design method and improving the manufacturing process, the number of parts of the automobile body can be reduced by about 20-30%, the mass of the automobile body is reduced by 15-25%, the torsional rigidity is improved by 60-70%, the vibration characteristic is improved by 30-40%, meanwhile, the bending rigidity and the impact load are enhanced, and the method is direct in process, more visual in design data and better in light weight effect.