This study proposes a diaphragm type Fabry-Perot (F-P) cavity liquid level sensor based on frequency-modulation continuous-wave laser interferometry. According to the frequency modulated continuous wave interferometry technology liquid level measurement system theory, the liquid level sensor is made, the liquid level measurement system is built, and its linearity, sensitivity, resolution and other tests are carried out. The experimental results show that the linearity of the liquid level measurement is 0.99998, the sensitivity of the liquid level measurement is 465 nm/mm, and the measurement resolution is 33.998 nm, the corresponding level value is 0.0731 mm, and has good stability, repeatability and hysteresis. Therefore, the designed frequency-modulated continuous wave optical fiber liquid level sensor can achieve high precision and stable liquid level measurement.