A new method is proposed for the online calculation of make-up electric power during voltage-loss of a three-phase watt-hour meter.The principle and advantages of the method are described，and an error analysis is made.Based on the classification of voltage-loss，a set of formulae of compensating electric power during voltage-loss of three-phase four-wire and three-phase three-wire watt-hour meter is given.Then the test data of the error of voltage-loss energy metering by using this method for compensating the active power under balanced three-phase voltage and unbalanced three-phase current load are given.This method can greatly improve the accuracy of make-up electric power of three-phase watt-hour meter under the field condition of unbalanced three-phase current and variable power factor during voltage-loss，if the grid voltage unbalance factor is within the range specified.