Abstract: It was summarized that normal function, mountain function, triangle fuzzy function and trapezium function is usually used to simulate a continuous membership function during fuzzy evaluation of engineering rock mass, and their analytical equations were presented accordingly. The establishing method of four membership functions for concrete engineering index was researched through using the uniaxial compress strength of rock mass as an example during fuzzy classification of rock mass. The membership degree of a uniaxial compress strength value was calculated by the above-mentioned four membership functions. It was found that there was a bigger difference among the membership degrees calculated by different subject functions. Five main indexes that influence rock mass stability were picked out through analysis. Based on data of the surrounding rock of a tunnel, the membership degree values of the uniaxial compress strength of rock mass, RQD, integrality coefficient and soft coefficient were calculated respectively by three membership functions, and the membership degree of structure surface was decided, through a membership degree value table, for the rock mass of roadway. Some difference among the membership degree values of the rock mass of roadway was found when the membership functions that were used to calculate the membership degree were different. Yet the classification result of rock mass is identical under different membership degrees from different membership functions. The researched result testified equivalence, adaptability and correctness of a common membership function in fuzzy. The theoretic base of choice a subject function in fuzzy classification for rock mass was suggested.