The invention discloses a user sag economic loss assessment method based on multi-quality loss function synthesis. The method comprises the steps of counting user sensitive equipment compositions in apower system, and calculating a synthetic voltage tolerance curve of any node; calculating an equipment failure rate, an energy index and voltage sag severity of the node under each voltage sag eventaccording to historical data, and calculating quality losses caused by the equipment failure rate, the energy index and the voltage sag severity ; according to calculation results, weighting the quality losses of four parts including the equipment fault rate, a voltage sag amplitude, the energy index and the voltage sag severity based on an entropy weight method; performing parameter estimation of a quality loss function according to the economic loss generated by voltage sag in a historical record; and performing voltage sag economic loss assessment of a user according to the sag amplitude and duration of current voltage sag to obtain the economic loss of the current voltage sag. The method has the advantages that the objectivity is higher and an assessment result is more accurate.