This paper proposes a new method to evaluate the voltage sag sensitivity (VSS) of the sensitive load by the combination of several probability density function models, which can fully describe the randomness of load sensitivity. Firstly, five common probability density functions were respectively used to build the mathematical model of voltage tolerance curve (VTC) of sensitive load in uncertainty region. Secondly, the VSS of sensitive load was respectively evaluated by five models, and the weighted sum of five models’ results was calculated by unbiased variance method. In the end, the proposed method was verified to be rational according to an application example.