Abstract:Assessment of surrounding rock stability is a key link to ensure the safety of engineering. Because of multifactor synthetic effect of rock mass internal structure and external environment,surrounding rock stability has not only randomicity but also fuzzy characteristic. Based on engineering fuzzy set theory(this theory has been founded by author—Chen Shouyu in 1990s),engineering method of variable fuzzy set is proposed. The base of the theory of variable fuzzy set is the relative different function;and variable characteristic of model parameter(index weight vector) is considered to assess the surrounding rock stability. It shows particularity and excellence of variable fuzzy set. The evaluation results are more reliable. The detailed calculational process of this method is proposed too. Offering a application and comparing the result with other methods,rationality and advantage of engineering method of variable fuzzy set has been verified. This method can well reflect every evaluating factor¢s influence on surrounding rock stability too. It can be used not only in assessment of surrounding rock stability but also in other engineering fields. This method is simple,so it is convenient for engineers and technicians to use in microcomputer.