A new method to study on the fault frequency of sensitive equipments based on the Maximum Entropy Principle due to voltage sags. Based on characteristics of voltage sag.The probability density function of VTC was determined by the Maximum Entropy Principle under the probability distribution of VTC is unknown. Using the cumulative method to calculate fault frequency of the sensitive equipment through the voltage sag characteristic parameters in the period of monitoring time.As a case study, the personal computer was simulated. The simulation results compared with existing methods show that the method needs no subjective assumption and the results accord with the practical situation when the probability distribution of VTC is unknown. And this method is accurate and adaptable.