Air-cooling systems and water-cooling systems of steam turbine differ much in their ends of exhaust, as a result that the method of back pressure control for ordinary water-cooling systems is not applicable to air-cooling systems. Therefore, the safety margin of back-pressure control is provided, based on which the fuzzy control approach is proposed for applying in direct air-cooled units. The back pressure of air cooled steam condenser is easy to be influenced by the local environmental temperature and the fans speed, so that a mathematical model is developed based on the traditional air-cooled units with off-design characteristics. It is verified by the simulation that the approach to a large extent can ensure the air-cooling system operating under most cost-efficient and safest conditions.