Frequency measurement is the basic function of power system measurement and of a control device and protection device. The rapidity and accuracy of frequency measurement will seriously affect the performance of power system measurement and the protection device, thus affecting the stability of the power system. In order to improve the rapidity of frequency measurement, a new frequency measurement algorithm is proposed. The algorithm can measure the frequency of the measured object in 5/4 cycle, and has strong real-time performance. First, the kernel function for calculating the real part of a Fourier series is phase shifted. Then the kernel function before and after phase shift is used to Fourier transform the original signal to obtain the real part of the signal. After that, the signal frequency is calculated by using the time difference between the zero crossing of two real part signals. Finally, the algorithm is verified by Matlab simulation and actual measurement of a protection device. The tests show that the proposed algorithm meets the accuracy requirements; the tracking speed of gradually changed frequency is greatly improved compared to that of existing algorithms. This work is supported by the Science and Technology Project of China Southern Power Grid Co., Ltd. (No. ZBKJXM20180500).