The invention provides a power grid harmonic measurement method capable of reducing sampling frequency. The method is characterized by carrying out downsampling on a power grid signal to obtain a first frequency spectrum, a second frequency spectrum, a third frequency spectrum and a fourth frequency spectrum, wherein the first frequency spectrum and the second frequency spectrum are same in sampling frequency; the sampling start time of the second frequency spectrum is delayed by tau sampling periods with respect to the sampling start time of the first frequency spectrum; the third frequency spectrum and the fourth frequency spectrum are same in sampling frequency; and the sampling start time of the fourth frequency spectrum is delayed by tau sampling periods with respect to the sampling start time of the third frequency spectrum. By carrying out frequency spectrum conflict detection on the first frequency spectrum and the second frequency spectrum or the third frequency spectrum and the fourth frequency spectrum, reconstructing the power grid signal by utilizing components having no spectrum conflict and updating the first frequency spectrum, the second frequency spectrum, the third frequency spectrum and the fourth frequency spectrum, and so on, reconstruction of the power grid signal is realized; limit of Nyquist sampling theorem is broken through under the condition of ensuring precision of harmonic measurement, and sampling frequency is reduced effectively; and the method is suitable for detection of high-frequency harmonics.