Abstract: In order to evaluate the chemical interaction degree between red beds rock and water,the properties of pH,EC,and some key materials are studied in laboratory;the idea of rock chemical-stability is advanced;the engineering criterion of red beds rock chemical stability is established,including pH,EC,percentages of key materials;according to the chemical stability criterion,the red beds rock is classified into three types: high chemical stability,middling chemical stability and low chemical stability;the chemical stability advice of quantifying evaluate and engineering design is provided for red beds projects.