Abstract: The recent reaearch result by authors indicates that the Hipparion (N2) red soil covered with loess and sand in northern China a regional expansive clay. It has high montmorillonite content and high swelling potential and hence has a harmful influence on engineering construction and is easy to undergo geologic harzards. This paper gives a result of comprehensive reserch on engineering characteristics of the Hipparion (N2) red soil and formation mechanism of harmful engineering behaviour in Abag Banner , Nei Mongol Autonomous Region.