The invention relates to a power signal amplitude detection method and system. Truncation processing is performed on the sampled signal sequences so that truncated signal sequences are obtained; a cosine function of the measured reference frequency and a sine function of the reference frequency are respectively multiplied by the signal sequences and the truncated signal sequences so that two sets of real frequency vector sequences and imaginary frequency vector sequences are generated; digital filtering is performed on the two sets of imaginary frequency vector sequences and the real frequency vector sequences so that two sets of imaginary number vector filtering sequences and real number vector filtering sequences are generated, and then two sets of imaginary number vector integral values and real number vector integral values are generated through integration; the two sets of real number vector integral values and the imaginary number vector integral values are converted into sequence amplitude and two phases, and the two phases are converted into frequency difference; and the sequence amplitude and the frequency difference are converted into fundamental wave amplitude of a power signal. The fundamental wave amplitude with relatively high accuracy can be obtained by application of the power signal amplitude detection method and system.