Dehiscence on arch dam is very common and worried by the engineers, so it is very important to find the essential cause of dehiscence and provide some useful countermeasures. The essential causes of asymmetric cracks in Si’de arch dam， analyzed by using structural mechanics method and 3D finite element method，is the asymmetric topography and the inadequate design of spillway and dam. The rational measures to deal with the cracks and to strengthen the dam have been provided, namely making upstream rock symmetric, mending the cracks under adequate temperature and pouring reinforced concrete slab on the upstream dam till grouting flat.
