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Serious problem for failure probability evaluation of equipment due to voltage sag is the uncertainties and mathematical expression of influencing factors. The uncertainties contained in voltage sag, equipment voltage tolerance and possible operation state are researched. The intension and extension uncertainties of influencing factors are used to distinguish their property. Stochastic and fuzzy variables are introduced to express intension and extension uncertainties. The mathematical models of influencing factors are established using stochastic and fuzzy models and a multi-uncertainty evaluation model is proposed also. In this method, maximum entropy principle is used to extract the probability distribution of voltage sag intensity and the determination principle of membership function of fuzzy safety event is used to determine the multi-uncertainty evaluation model. As a case study, personal computer is simulated. The simulation results show that this method is correct and viable and it can be easily used in other fields.