This paper proposes a method for measuring the fundamental frequency of power system. First, the valid order of the reconstructed signal is determined by the increment of singular entropy and its incremental differential. Then, the grid voltage signal is reconstructed through the reverse process of singular value decomposition. By this way, the fundamental signal is extracted, the noise and the disturbance is eliminated. Finally, the fundamental frequency of power system is calculated by TLS-ESPRIT. The result of simulation tests proves that the method is effective, the value of the fundamental frequency is estimated more accurately, in the cases such as noise, frequency offset, harmonic with instantaneous impulse, harmonic with oscillatory transients.