Langshansha submerged dike in Nanjing Yangtze river deepwater channel phase I project contains 1 869 meters semi-circular composite dike.The large-scale semi-circular prefabricated structure is a shaped thin-wall opening large components.The structure thickness of concrete cover is the key and difficult part of prefabricated engineering.This paper analyzes the influential factors of the thickness of concrete cover for the semi-circular prefabricated structure from many aspects,and discusses the means of improving the qualified rate of the thickness of concrete cover.