Abstract: Based on the relevant theories of fuzzy fault tree, the index of fuzzy cost importance is proposed to assess the effect of each risk on the total construction cost during shield tunnel construction. The assessment marks of experts are modified to triangular fuzzy numbers using the confidence index. Furthermore, an accurate calculation method for the multiplication of fuzzy numbers is derived, and the fuzzy cost importance of each risk is obtained according to the representation theorem. By analyzing the index of fuzzy cost importance, the risks that should be avoided primarily before the construction can be obtained. The proposed method provides a scientific and reliable means for the risk assessment of shield tunnel construction cost.