The expandable casing technology using in geo-engineering drilling is being studied. This paper introduces study status of the expandable casing technology and the type of expandable casing. This study determines selection principles of the expandable casing material and the expansion tool shape, analyzes the expansion mechanism of expandable casing and establishes the construction process. Wall protection technology by expandable casing can be applied in geo-engineering drilling in China and increase the means of wall protection technology of geo-drilling.