To solve the problem of asynchronous frequency-fixed and sampling system’s complex computation and weak real-time for power system frequency measurement, a practical real-time frequency measurement algorithm is proposed. This method is based on the interpolation principle of periodic zero-crossing point, the frequency estimated value is calculated by digital filtering of FIR band-pass for A/D sampling discrete sequence and by interpolating for adjacent zero-crossing point with the same direction. The signal under test in power system is simulated and vertified under three different circumstances: excluding harmonic, including second to sixteenth harmonic and the random noise. The simulation result is compared with that of Discrete Fourier Transform (DFT) method. The result shows that this method is superior to DFT in anti-interference and measurement accuracy, it can satisfy the requirement of frequency measurement for power system due to its simplicity, strong practicability, high instantaneity and measurement accuracy.