Artificial gravel or sand islands were constructed in shallow water of the Arctic Sea to drill explorative or productive wells. However as increasing the water depth of the site, it became difficult and expensive to complete an island in one season. So the mobile islands such as Mobile Arctic Caisson (MAC) have been developped in place of artificial gravel or sand islands. In this paper, the authors studied the design criteria of a mobile caisson island, and investigated the structural strength analysis with its application to MAC, which was constructed in our yard. In the structural analysis of a mobile caisson island, the effect of sand shall be the most important. Usually the sand shall be treated as simple lumped springs distributed on soil face and bottom of the caisson. But in this method, it is difficult to obtain the accurate results, because the calculation of the spring constant is very difficult, and it is impossible to be taken into account of the interaction of each spring. So in this paper, to obtain the accurate results, the authors analyzed that interation and contact problem of the sand and the steel structure with finite element model.