Abstract:Duality element relative fuzzy evaluation method was briefly introduced. The two forms of membership function that express membership degree of basic element of rock mass were researched at first. For some indices,which are continuous in math,their membership functions can be expressed through continuous functions,such as normal function,triangle fuzzy number and sine-wave function and so on. Degree of membership of discrete indices can be got through table of membership degree. Elements that effect rock-mass stability are analyzed. Five indices of these elements were picked out as the foundations of rock mass stability classification,and membership function of each index of the five above-mentioned indices was established. Duality element relative comparison method was adopted to assure relative weight between two elements. It was introduced how to use duality element relative comparison method when it was adopted in rock mass stability classification. The engineering geology of an underground rock mass engineering was introduced at last. Stability grade of surrounding rock of the underground engineering was analyzed by using the classification method. The primary analysis progress and results were introduced and the analytical results were validated during construction of engineering. The analytical results show the classification approach of the rock-mass stability researched by the authors is reliable and practical.