Abstract: A Bentonite contains about 40 per cent of montmorillonite.Its physical and chemical properties are discussed.The clay treated with the traditinonal way cannot meet demands for the well drilling mud materials.Based on the specific characteristics of the clay,this clay was treated with a composite additive among which is soda ash,activeted magnesium oxide,and natural chainline macromolecule compound,and synthetic macromolecule polymer.The results were shown that the making mud properties of modifying clay were improved significantly.All its technical index were up to the international standard,ISO 13500∶2008.