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• Laser frequency drift characteristics, such as fluctuation range and frequency, influence the performance of the closed-loop frequency-locked module. • A novel measurement system is proposed, based on the Fabry–Pérot cavity and sinusoidal modulation of cavity length and demodulation by LIA. • This measurement scheme can be applied in other fiber sensor systems to evaluate the effect of laser frequency stability. In this study, to measure the frequency drift characteristics of the narrow-linewidth laser, we provide a method with high accuracy and adjustable measuring range that can be applied in a resonant optical gyro (ROG) system. In an ROG system, the laser frequency drift characteristics, such as fluctuation range and frequency, influence the performance of the closed-loop frequency-locked module. To measure the above performance of the laser, we proposed a novel measurement system based on the Fabry–Pérot cavity. We proved the feasibility of the scheme through theory and simulation. Then, the calibration of the experimental device and the measurement of the tested laser were completed. The measurement accuracy of this method is 1.2 kHz, and the measurement range reaches ±60 MHz, which can be adjusted by the modulation index. The results show that the laser has a frequency fluctuation of 30 MHz in the stabilization stage, and there is mainly a low-frequency component under 5 Hz in the frequency domain of the laser frequency fluctuation. Furthermore, this measurement scheme can be applied in other fiber sensor systems to evaluate the effect of laser frequency fluctuation on system output performance. [ABSTRACT FROM AUTHOR]