In the clay soil more or less amorphous free ferric oxide is present. Properties of clay soil are effected by the amorphous free ferric oxide and its crystals. In general the amorphous free ferric oxide effects the physical, hydrographical and engineering geological properites of clay soil. In this paper, the variation in properties of clay soil is studied by experiment in the process of dehydration. The authors hope this study is of benefit to geotechnical testing, geotechnical investigation and soil physics. The main results are as follows: (1) Liquid limit, plastic limit, and shrinkage limit in clay soil increase with increasing content of amorphous free ferric oxide, but in the process of dehydration of amorphous free ferric oxide this strengthening action decays. (2) In the clay soil amorphous free ferric oxide enhances its shrinkage. In case amorphous free ferric oxide was crystallized, the strengthening action decays or vanished, in this time strengthening action turned to restraint action on shrinkage. (3) In the process of dehydration, amorphous free ferric oxide forms structural connection, mechanical strength is enhenced by this connection in the clay soil, and the connection restrains expansion of clay soil.