The invention relates to a frequency detection method and system of a power signal. The method comprises: inverted outputting is carried out on a forward signal sequence obtained by sampling to generate a deconvolution sequence; the forward signal sequence and the deconvolution sequence are cut into short to generate two groups of cutting signal sequences; a cosine function and a sinusoidal function of a reference frequency are multiplied by the forward signal sequence, the deconvolution sequence, and the two groups of cutting signal sequences to generate four groups of real-frequency vector sequences and imaginary-frequency vector sequences; digital filtering and integral operation are carried out on the four groups of real-frequency vector sequences and imaginary-frequency vector sequence successively to generate four groups of imaginary number vector integral values and real-number vector integral values; and the four groups of imaginary number vector integral values and real-number vector integral values are converted into four phases and then the four phases are converted into initial phases and cut-off phases, differences between the cut-off phases and the initial phases are converted into all-phase differences, and a preset sampling frequency, a preset sequence, and the all-phase differences are converted into power signal frequencies. With the method and system, signal frequencies with high accuracy can be obtained.