Lightening the structure weight is one of the most important targets of the aircraft design. It could be realized by applying the composite material instead of the metal and optimizing the structure. An optimization design is done for a high aspect ratio composite wing in this paper. The design variables are the position of the spars and the composite layup. The target is to minimum the weight. And the constraint conditions are the structural strength and stiffness. The optimization is done through genetic algorithm with Matlab calling Patran. The composite layup library is built to solve the problem that the number of the composite layup variables is too huge. The result shows that this optimization method is effective and it is useful in engineering design. At the same time, the composite layup library also can be widely used in other works related with composite layup optimization.