The weak sandwich formed during construction of the newly filled coral sand foundation is characterized by broad distribution,random thickness,poor engineering property,and shows no apparent rule,thus it brings great difficulty and challenge on the engineering construction.So,we carry out an in-door experiment to study the characteristics of the weak sandwich.The results show that the weak sandwich is mainly made of silty soil and has a high plastic limit；its shear strength is higher that of the natural powder sand；the more the content of the fine particle(with particle size of less than 0.075 mm),the higher water content in the soil and the smaller the maximum dry density.