The present invention discloses a design method for reducing a cross section of a wind farm output wire. The method comprises the work of preliminary design, reduction coefficient calculation of a cross section and verification. First, based on a design capacity of a wind farm, preliminarily selecting a wire cross section according to economic current density; then, according to environmental characteristics of the wind farm, in combination with a mathematical relationship between a wind speed, along with environmental factors, and a wire current capacity, and by separately using a screening loaded mode and a Bayesian analysis method combining with Markov Chain Monte Carlo, performing effective reduction on the primarily selected wire cross section, so as to determine a wire design cross section of a output wire; and finally, performing verification according to a continuous limit conveying capacity, short circuit thermal stability and a depressurization condition under a heating permitted condition. The method disclosed by the present invention can combine with an actual working condition to perform design and selection on a wind farm output wire, and compared with a current selection method, a cost performance can be further improved when safety is ensured.