Based on the layout of main buildings, structures and the air-cooled island in a practical 135 MW power plant project, the mathematical models for flow and heat transfer of direct air cooled condenser are developed. Using the numerical heat transfer (NHT) software called FLUENT, thermal performance of air cooled condenser under typical operation conditions of steam turbine, especially turbine rated power load (TRL) mode in summer, is simulated. The main factor which influences the heat transfer performance of air cooled condenser is the backflow or recirculation of hot air caused by the environmental strong wind. The simulating results, including the flow field and temperature field, are analyzed. It offers good help for the optimal design of direct air cooling system.