Abstract: The method of parameter optimization is adopted to carry out lightweight design of the aluminum alloy carbody of a type of EMU. The first-order vertical bending modal frequency and first-order rhombic modal frequency of the carbody are improved through structure optimization. Then, the weight of the carbody is reduced and the requirements of the optimization objectives are met through parameter optimization means such as reasonable distribution of profile skin thickness, form optimization of profile internal ribs and adjustment of profile width.