An extended top of the roof can induce the upflowing wind which flows close to the wall and in this way it increases the intake airflow rate in the air gap. A model was set up to save energy with the consideration of a suitable thickness of the air gap and a suitable length of the extended top. The Computational Fluid Dynamics (CFD) was employed to simulate the wind field in the ventilated roofs with extended top and the cases were carried out according to Changsha’s climate parameters in China. The results show that the extended ventilated roof works very well in summer. The air velocity inside air gap and airflow rate increases with the increasing thickness of the air gap, but for the length of the extended top, it almost changes nothing when its length reaches a certain value. With the same extended top, the thicker the air gap is, the more energy it can save. In other hot summer regions an optimized ventilated flat roof can also be gotten in the similar way mentioned in this paper.