This paper presents a method for multi-objective optimization design of wind turbines at the system level through a coupled blade-tower model, aiming at finding the coupling effects between blade and tower and improving the wind turbines' performances. The formulation and implementation that enable the aerodynamic and structural integrated design of the blade and tower simultaneously are detailed. The maximum annual energy production (AEP) and the minimum wind turbine mass are taken as two conflicted objectives. Main aerodynamic and structural parameters of the blade and tower are employed as design variables. Various design requirements including stress, strain, deflection, vibration and buckling limits are considered as constraints. The blade element momentum (BEM) theory combined with the finite element method (FEM) are applied to evaluate the aerodynamic performances and structural behaviors of the turbine. Moreover, the non-dominated sorting genetic algorithm (NSGA) II is used to achieve the best trade-off solutions between the objectives. To show the efficiency and reliability of the method, a commercial 1.5 MW wind turbine is used as a case study. Satisfactory results that can both increase the AEP and decrease the mass are obtained, which are superior to the results achieved by optimizing the blade and tower separately. (C) 2020 Elsevier Ltd. All rights reserved.