The invention relates to a power system frequency measurement method and system. Firstly, the sampling signals of the power system are subjected to low pass filtering, and the high-frequency interference signals are eliminated; the obtained low-frequency signals are subjected to sinusoidal Fourier calculation, and phasors of three moments with a difference of a 1/m cycle are obtained to eliminatethe influence of low-frequency harmonics, and m is no smaller than 3; and finally, according to a phasor frequency measurement method, the real-time frequency of the power system is obtained. The low-frequency signals of the power system are processed, the sinusoidal Fourier is used to eliminate the influence of low-frequency harmonics, the frequency result of the power system which is obtained through calculation is more accurate, and in comparison with the method used in the prior art, a lot of iteration work is not needed, the calculation method is simple and reliable, popularization is easy, and the practical application value is strong.