The invention relates to an equivalent harmonic impedance method for calculating the single-point equivalent of a power grid. The method comprises the steps of firstly, acquiring the bus voltage at a point of common coupling and the current of a current branch of a focused harmonic source, conducting the fourier transformation and decomposing the voltage and the current to obtain a harmonic voltage and a harmonic current; secondly, grouping every five pairs of the bus harmonic voltage at the point of common coupling and the harmonic current of the infusion bus of the focused harmonic source as one group to establish a complex domain partial linear model, and figuring out an equivalent harmonic impedance estimated value, an equivalent harmonic voltage estimated value and an estimated error thereof for each group; on the condition that the minimum value of the estimated errors of all groups is greater than a given error limit value, re-selecting data groups for calculation; otherwise, figuring out the mean value of equivalent harmonic impedance estimated values corresponding to the minimum value when the minimum value is smaller than or equal to the given error limit value, namely the equivalent harmonic impedance of the power grid in the single-point equivalent state. According to the technical scheme of the invention, the equivalent harmonic impedance of the power grid in the single-point equivalent state can be obtained accurately.