The invention relates to a power system frequency measuring method and a power system frequency measuring device. First, a current signal to be measured is sampled, and an obtained sampling signal is made discrete; then, sampling matrixes [In] and [In-1] of the current signal to be measured at the time n and at the time n-1 are listed based on the discrete sampling signal, a reference frequency f(reference) is set, a reference voltage signal u(t) is constructed based on f(reference), and u(t) is made discrete to obtain a reference matrix; and finally, phase values corresponding to the time n and the time n-1 are obtained based on characteristic roots of matrixes [In]+[U] and [In-1]+[U], and a calculation frequency f(calculation) is solved. According to the invention, QR decomposition and similarity transformation are performed on the sampling matrixes constructed with a reference signal and the signal to be measured, the phase of the signal to be measured is obtained based on the characteristic roots of the corresponding matrixes, and the frequency of the signal to be measured is worked out based on the phase difference between former and latter moments of time. Compared with the prior art, the calculation process is simplified, the amount of calculation is reduced greatly, and the frequency of a sinusoidal signal can be calculated in a power-frequency data window quickly and accurately.