Due to the influence of environmental wind on direct air-cooling condenser arranged high in the sky, the hot air recirculation and intrusion phenomenon easily occurred, which would affect the security and economics of the unit. Therefore, this paper proposed underground ventilation mode for air cooling island. The physical model of air cooling island with underground ventilation in a 600 MW direct air-cooling unit was established. The air flow filed and temperature filed of air cooling island with two-duct underground ventilation was numerically simulated with using Simple algorithm and standard k-ε model in Fluent. Then, the ventilation rate of air cooling condenser with underground ventilation or overground ventilation was calculated, and the impact of wind speed on the heat transfer efficiency and pressure of condenser under dominant wind direction was analyzed. The calculation results show that, under the influence of environment wind, the condenser performance of air cooling island with underground ventilation is better than that with overground ventilation, and there is not hot air recirculation and intrusion phenomenon in any wind speed. When environmental wind speed is over 4 m/s, under the same conditions, the ventilation rate of air cooling island with underground ventilation is greater than that with overground ventilation, and the heat transfer efficiency is higher. When environmental wind speed exceeds 8 m/s, the condenser pressure of air cooling island with underground ventilation is about 7~11 kPa lower than that with overground ventilation.