Abstract: This paper introduced the current status of the foundation design of offshore wind power projects under the extreme marine environment in the southern sea of China, and described the structural types of large diameter single pile. Next, this paper introduced the important influence of the top flanges to the wind power. Assuming the top flange has concave defects, this paper gave two main treatment methods, which has been introduced and analyzed by the finite element method and empirical formula in this paper. According to the analysis results, the two methods could give good solution of the flange defects, which could provide directions and suggestions for the subsequent treatment of flange depression defects.