Abstract: Deep excavation engineering is characterized by vast construction investment, many unpredictable factors, complex construction technology and great society influence. Therefore high risks would be induced in the deep excavation engineering such as economic and technological risks during construction. It is difficult to determine the risks with a precise value because they are objective, inevitable, various in kind, dynamic and fuzzy. A methodology of giving a range using the fuzzy membership function can describe the risks rationally and effectively. Firstly, experts give estimated values for the probability and loss of deep excavation accidents according to probability and loss rank. Secondly, the assurance degree of the estimated values is determined according to the complexity of work, the skill of experts etc, and the proper fuzzy membership function is chosen according to the assurance degree. Thirdly, the risk range is determined based on the fuzzy membership function. Finally, compared with the risk range and acceptance criteria of risk, the risk rank of an excavation can be obtained. By use of the risk assessment methodology, the risk rank of a given deep excavation is obtained.