The invention discloses a curve fitting method for duration of multiple voltage sags, and a voltage sag severity assessment method. The curve fitting method comprises the steps: obtaining the change rule of the voltage value of a power grid with time in a unit time, and obtaining a voltage change time sequence curve; obtaining the number of voltage sags of the voltage in the unit time and the data of each voltage sag according to the voltage change time sequence curve, wherein the data of each voltage sag comprises the time period of the voltage sag and the voltage value corresponding to the voltage sag; taking a plurality of voltage values Vi between the maximum voltage value Vst and the minimum voltage value Vmin of each voltage sag; calculating the time duration values corresponding to the plurality of voltage values Vi; superposing the time duration of all voltage sags corresponding to the same voltage value Vi, so as to obtain the total time duration under the same voltage value Vi, and to carry out the fitting of the total time duration curve according to the total time duration corresponding to the plurality of voltage values Vi.