Abstract: Air Cooled Condenser (ACC) is the key equipment of the cold end system in power plant and its performance has a direct impact on the safe and economic operation of power unit. The physical and mathematical models of a 600MW unit are established. The 3D numerical simulation of ACC system with the spray humidification device at the bottom of air cooled condenser is given by using the FLUENT for the economic operation of unit in summer. The results indicate that both the inlet and outlet air temperature of ACC decline remarkably through the spray humidification device and improve the heat transfer in ACC. Both the heat exchange efficiency and the vacuum in the ACC are improved obviously. The method can keep the power unit operating safely and economically during the hot summer season.