Abstract: According tO the principle of grey correlation analysis, the reference number series are composed of the indices of the adjoining rock to be classified in underground constr-notion. Compared series are composed of the indices of the classification standards for the adjoining rock in underground construction. A grey optimal theory model for the stability classification of adjoining rock is established through the extended principle of minimum  square.Combined with practical examples, its application is described and the results are compared with those of fuzzy mathematics.