The invention relates to a car body light-weight design method. The method comprises the steps of building a car body finite element model; 2, building a car body modal, bending and torsional rigidityanalysis model, and calculating and obtaining modal, bending and torsional rigidity values; 3, building a sensitivity analysis model and performing solution calculation; 4, performing finishing according a sensitivity analysis model calculating result, obtaining a sensitivity analysis result of a car body metal plate corresponding to the design variable; 5, building a car body light-weight optimizing calculation model, and performing solution calculation; 6, performing extraction on the calculation result, and modifying a thickness value of some metal plates so as to meet the requirement forengineering, and forming a novel BOM table; 7, judging whether the BOM table reaches a light-weight design target or not, if yes, completing designing, and if not, repeatedly executing the step 6 tillthe light-weight design goal is reached. Compared with the prior art, the requirement for the car body rigidity is met effectively, the car body weight is lowered, the high engineering practicabilityis achieved, and the applicability is high.