The external flow filed of the air-cooled island is numerically simulated by CFD for a 2×600 MW direct air-cooled unit. For the wind speed higher than 7 m/s, the combination of hot air recirculation and downdraught of the air cooled condenser results in the reduction of heat exchange efficiency. Therefore, fixing lower windbreak with curved surface upon direct air-cooled condenser is considered. Furthermore, the effects of lower windbreak height on average heat exchange efficiency and flow capacity of fans are discussed. It is shown that the optimum height of the windbreak is 85 m, where the fan flow and heat exchange reach to the maximum. Especially the average heat exchange efficiency of the first two lines of fans increases 1703% than that without the lower windbreak. It provides theoretical evidence for the structural design and optimization of the air-cooled island.