A finite element model of a 1.5 MW wind turbine blade was established, and sensitivity analysis of key structural parameters was carried out. The results show that the ply angle has the greatest influence on the structural performance of the blade, with the width and the layer thickness of the spar cap coming second, and that the layer locations of the spar cap and the position of the shear web have the least influence. Based on the sensitivity analysis, an optimization design mathematical model pursued with respect to minimum mass of the blade was developed. The width, layer number, and layer location of the spar cap, and the position of the shear web were employed as the design variables, while the demands of strength, stiffness, and stability of the blade were taken into account as the constraint conditions. The optimization design of the blade was carried out by combining the genetic algorithm and the finite element method. Compared with the original design, the optimization design results achieve a decrease of 9.8% of blade mass, and the strain distribution of the structure is more reasonable, without occurrence of resonance, showing that the method used in this study is efficient.