The invention relates to a power amplitude measuring method, which belongs to the technical field of power system control and protection. The method provided by the invention comprises the steps that a frequency tracking unit is used to carry out power sampling to ensure that a sampling point is synchronous with the actual frequency of measured power and the number of sampling points of each cyclic wave is equal to or greater than 2 times the number of harmonics; based on a discrete Fourier principle, the concept that cosine and sine coefficients are fixedly corresponding to sampling points is used to iteratively calculate sine and cosine components of each sampling point; sine and cosine components of each sub-harmonic of power are calculated; according to sine and cosine components of each sub-harmonic of power, the amplitude of each sub-harmonic is calculated; the amplitude of each sub-harmonic is equal to the root-mean-square value of sine and cosine components of each sub-harmonic; and the power amplitude is measured. On the premise that the calculation accuracy is not reduced, 2N to 2 multiplication operations are reduced than a conventional Fourier algorithm. Rapid calculation of high-accuracy amplitude is realized. The power amplitude measuring method is suitable for occasions needing real-time and high-accuracy magnitude calculation.