Based on the plate tectonics and Hthosphere dynamics. the earths crust could be divided into different-scale landmasses according to a series of active fault systems or crustal defomation zones as boundaries, in which relative stable landmasses are called safety islands. The concept of stable parameter (△f) is suggested to express the stable state of a safety island quantitatively. Thus, it is a new method introduced for selecting and evaluating regional stability or safety of a site for important engineering project. As an example, it is applied to optimize seeking and evaluating dam sites in Heishanxia area of Huanghe River, and it is concluded that both Xiaoguangyin and Daliushu sites can be selected for high dams.