Cement-soil mixed piles reinforcement is considered to be an effective way to improve the bearing capacity of the monopile foundation for offshore wind turbine in soft clay. In this study, a numerical model is developed to study the behaviors of the pile in cement-soil mixed piles reinforced soil. Factors influencing the reinforcement efficiency of cement-soil mixed piles method, including the reinforcement width, reinforcement depth, and the layout configuration, are investigated. Considering the improvement in both bearing capacity and deformation characteristics, a configuration with inner and outer rings and connecting walls with a replacement ratio of 66.15% is thought to be the optimum reinforcement configuration, while the structure stability and scour protection are also benefited. A method called the equivalent pile diameter method is proposed for the design of cement-soil mixed piles. Equations based on single parameter and multi-parameters for the design of the cementsoil mixed piles reinforcement are presented. This study and the proposed method are aimed at simplifying and facilitating the design of the pile foundation in offshore soft clay.