Abstract: Single probability in traditional fault tree analysis was used to represent each basic event.However,it was unrealistic to evaluate the occurrence of a top event with a crisp value without considering the inherent uncertainty and imprecision of each basic event.The fuzzy set theory could be used to deal with this kind of phenomenon.A triangular fuzzy number was introduced to represent the failure probability of a basic event.Fuzzy fault tree algorithm was developed by the cut method.The fuzzy failure probability of SMW retaining structures in deep excavations was calculated by a fuzzy fault tree.Main basic events affecting the occurrence probability of the top event were verified by a sensitivity analysis,which could be applied in the risk analysis of SMW retaining structures in deep excavations.Compared with the traditional fault tree analysis,the fuzzy fault tree method could not only reach the same goal as the traditional method but also get the failure possibility distribution of SMW retaining structures in deep excavations.