Abstract: Due to factors such as power system asynchronous networking, isolated grid operation, low-frequency oscillation, etc., the problem of system frequency stability becomes prominent, which may lead to incorrect protection actions. This paper first considers the influence of the initial phase angle oscillation of the Fourier algorithm caused by frequency fluctuations and the change of the system impedance on the measured electrical quantities, and derives the boundary formula of the fault current amplitude-frequency characteristics measured by the Fourier algorithm at non-power frequency. Then, the formula of sensitivity coefficient and protection range varying with the amplitude-frequency characteristics of fault current is derived. Finally, a new method for calculating the frequency that makes the protection range of the three-stage current protection of the line reach the critical value is proposed, and the action of each protection section when the frequency changes is analyzed. It can be used to analyze the ability of the line three-stage current protection to withstand frequency fluctuations. The fault current amplitude-frequency characteristic boundary formula is used to correct the measurement error of the Fourier algorithm of the three-stage current protection when the frequency fluctuates, and propose the new algorithm for fault current measurement at non-power frequency.