The effect of different level of cloud computing applications on different IT decisions vary differently, and the mechanisms of IT decisions will change at the same time. This paper has analysis the impacts of three service modles of cloud computing on IT decisions and put forward a new IT decision-making mechanism framework. Then, this paper use an case about a oversea enterprise which using cloud computing successfully to analysis and test the decision-making mechanisms framework this paper has proposed before. This paper only add a new dimension of three service modles of cloud computing to the original framework, but also provide a regime framework for enterpeise applying cloud computing.