Currently, the cross-section optimal design of overhead transmission lines for wind farms is on the basis of typical environmental parameters, which neglects the dynamic changes of weather conditions (like wind speed) and still has potentials to dig out transmission capability. In order to furtherly optimize the cross-section, this paper establishes mathematical model to characterize the optimal cross-section design process of overhead transmission lines for onshore wind farms and proposes an method which can be applied to optimize the cross-section design of overhead transmission lines. In which, the cross-section of transmission lines is designed and selected optimally considering the match between dynamic ampacity increasing of overhead transmission lines and wind turbine output characteristics on the basis of reginal historical meteorological data. Finally, the proposed method is verified by case study. Results indicates that the proposed method efficiently decreases the cross-section of transmission line, saves wire material and improves economic performance of electrical design for land wind farms.