This paper introduces detailed design of a high accurate frequency-measuring algorithm for smart grid feeder terminal units.The algorithm can be used for protection purpose.Firstly，the traditional Fourier algorithm is improved to improve its flexibility，and the1/10time phase difference of fundamental wave base on the phase length of0.2πis calculated.At the same time，using the mean error minimization algorithm and iteration method，a lower fixed sampling frequency of4kHz can be set. It can be completed easily on hardware and is not limited by synchronous sampling.Whether the steady power signals include a large number of harmonics and white noise or the transient power signals，in the case of frequency and amplitude changing，the proposed method is compared with current frequency-measuring algorithms，the results of simulation and experimentation show that the algorithm is accurate and does not have a big computation，and has an obvious advantage in the case of tracking time and overshoot error，fitting for actual application of measurement and control device for smart grid distribution automation perfectly.