The stability of rock mass is affected by many undetermined factors having random and fuzzy behavior. And it belongs to the fuzzy probability category. To assess the stability of rock mass with classical fuzzy comprehensive assessment methods may lead to incorrect results. Based on the fuzzy probability theory

a new comprehensive assessment method for the stability of rock mass (Fuzzy Probability Evaluation Method) is presented in this paper. The fuzzy weighted values are established to avoid the uncertainties while determining the index weights. Five factors which affect the stability of rock masses are picked out. For each of them a subordinate function and fuzzy weighted value are established. An example of underground rock mass engineering is introduced at last. Stability grade of surrounding rock of the underground engineering is analyzed using the Fuzzy Probability Evaluation Method. The analysis process is introduced and the results are compared with other methods. It has been proved that the method of this paper is reliable and practical.