Abstract: Taking the 1 000 MW direct air-cooling unit as an example, numerical simulation was carried out on external flow field of its radiator, so as to analyze the influence of following factors on external heat-transfer and flow characteristics of the radiator, such as the head-on wind velocity, ambient temperature, fin spacing and fin thickness, etc. Results show that with the rise of head-on wind velocity, both the external heat-transfer coefficient and flow resistance increase significantly; ambient temperature affects little on both the external heat-transfer coefficient and flow characteristics, but influences much on the total heat release. By enlarging the fin spacing, the external heat-transfer coefficient may be raised and the flow resistance may be reduced, however, this will lead to reduction of heat-transfer area per unit length of tube, resulting in decreased total heat release. For a fixed head-on wind velocity, there exists a reasonable fin spacing and fin thickness, which respectively increases and reduces with rising head-on wind velocity.