For many dam projects in China, the 50-year designed life time is coming to an end. It is urgent to study the theory and method to evaluate the dam service life. In this paper, firstly, the probability theory of fuzzy event and time-varying effect theory are used to analyze the time-variety of various risk factors in the process of dam operations. A method is proposed to quantify the above time-variety and a model to describe the fuzzy time-varying risk probability for the dam structure is also built. Secondly, the information entropy theory is used to analyze the uncertain degree relationship between the characteristic value of membership function and fuzzy risk probability, and a mathematical method is presented to calculate the time-varying risk probability accordingly. Thirdly, the relation mode between time-varying risk probability and service life is discussed. Based on this relation mode and the acceptable risk probability of dams in China, a method is put forward to evaluate and forecast the dam service life. Finally, the proposed theory and method are used to analyze one concrete dam. The dynamic variability and mutation feature of the dam risk probability are analyzed. The remaining service life of this dam is forecasted. The obtained results can provide technology support for the project management department to make treatment measures of engineering and reasonably arrange reinforce cost. The principles in this paper have wide applicability and can be used in risk analysis for slope instability and other fields. For many dam projects in China, the 50-year designed life time is coming to an end. It is urgent to study the theory and method to evaluate the dam service life. In this paper, first, the probability theory of fuzzy event and time -varying effect theory are used to analyze the time-variety of various risk factors in the process of dam operations. A method is proposed to quantify the above time-variety and a model to describe the fuzzy time-varying risk probability for the dam structure the information entropy theory is used to analyze the uncertainty degree relationship between the characteristic value of membership function and fuzzy risk probability, and a mathematical method is presented to calculate the time-varying risk probability accordingly. Thirdly, the relation mode between time-varying risk probability and service life is discussed. Based on this relation mode and the acceptable risk probability of dams in China, a method is put forward to evaluate an dcast the dam service life. Finally, the proposed theory and method are used to analyze one concrete dam. The dynamic variability and mutation feature of the dam risk probability are analyzed. The remaining service life of this dam is forecasted. The obtained results can provide technology support for the project management department to make treatment measures of engineering and reasonably arrange reinforce cost. The principles in this paper have wide applicability and can be used in risk analysis for slope instability and other fields.