Abstract: The size optimization for a cantilever I-beam under distributed loads is based on the variational method, and the width distribution of an edge strip under the strength and stiffness constraints is used as an example to demonstrate the use of the variational method. The optimization criterion method and the MIGA (multi-island genetic algorithm) with the FEM (finite element method) are discussed. An engineering cantilever I-beam model of the wing structure is presented to illustrate the three methods. It is shown that the variational method can greatly improve the effciency compared with the other two methods, and also the exact solutions of the optimal size distribution can be obtained, which can be used in the wing structure design during the preliminary design phase.