Due to the high J C of the superconductor, the volume and the weight of the generator can be significantly reduced if the superconducting coils are used. This paper has designed a 40-pole racetrack excitation winding for the 12MW offshore wind turbines generator. The excitation winding are wound by Nb-Ti superconductor, running at the temperature of 4.2K. In order to ensure the stability and reliability of the magnetic field, some optimizers for the design of the excitation winding are adopted. As a result, the fundamental magnetic field at the air-gap center of the armature winding could reach 2.1T, thereby the 12MW power generation could be achieved.