This article investigates the structural optimization of a 20-MW offshore wind generator to minimize the torque ripple and mass. The surface-mounted permanent magnet topology will be chosen for the generator. In addition, the double-layer armature winding configuration will be utilized. The direct-drive technology will be adopted to increase the reliability of the offshore wind energy conversion system. The multi-objective optimization of the generator is carried out based on the particle swarm algorithm. Results show that the torque ripple of the optimal model is significantly reduced to as much as 0.41% at full-load conditions making the configuration suitable for the high-power offshore wind generator.