Abstract: The form of an island arc is shaped by the rock expansion of back arc basin in long axis direction, which makes axis of back arc basin deformed, that is, it makes island arc bend down to subducted zone direction and forms arc?shaped island. Such a rock expansion attributes to the fact that dehydration of subduction oceanic crust results in the bottom of continental crust on top of the dehydration band (back arc basin long axis direction) to be melted (base invasion), forming arch melting surface, above which the rocks appear to be warmed up and expanded in differentiation. The rock expanding length of back arc basin in long axis direction is directly proportional to the radian of an island arc. The arc length of island arc is equal to the length of back arc basin long axis plus the length of the rock expansion. So, the offset distance and arc length of an arc island can be calculated according to the length of rock expansion, similarly, the length of rock expansion can be calculated by the offset distance or the arc length. Finally, on the basis of rock expansion length and rock lithology, the added temperature value of rocks on rock melting surface, the thickness of continental crust bottom melting (base invasion) area and subsiding depth of the back arc basin are all calculated