Abstract: According to the randomness and fuzziness of the factors which affected the stability of underground structure,firstly,by introducing the theory of triangular fuzzy number,the probability distribution function of physical and mechanical parameters of underground structure was established.Then,based on the studies on methods to decide the functional function of analysis of fuzzy reliability for underground structure,a method to determine the range interval of functional function was founded by introducing interval-truncation approach and rules of operation of interval numbers,and the interval expansion of functional function was avoided.Finally,through studying the method to measure fuzzy probability of reliability of underground structure,a method to determine the distribution curve on probability of fuzzy reliability index and the failure possibility of underground structure was developed,thereby.A model to analyze the reliability of fuzzy probability of underground structure was brought forward,and its rationality and feasibility were validated by the computation and analysis of a practical engineering project.