To accurately detect liquid level and pressure, a fiber optic diaphragm sensor and the measurement system are firstly prepared in this article. The sensor consists of fiber Bragg grating ( FBG), elastic diaphragm, microcavity, FBG fixture and silica gel tube. Secondly, the theoretical model of the sensor to detect the liquid level and pressure is formulated, and the response characteristics of the sensor to the liquid level, pressure and temperature are experimentally studied. Finally, to study the anti-interference ability of the sensor to changes in the external environment, the effects of temperature, inclination angle and liquid level velocity on the measurement results of the sensor are experimentally studied. Results show that when the liquid level velocity is in the range of 10~ 100 cm/ min, the sensor′s inclination angle is in the range of - 30° ~ 30°, and the tested liquid temperature is in the range of 20 ~ 60℃ . A linear relationship between the sensor output and liquid level 0~ 220 cm and pressure 0 ~ 22 kPa is obtained. The measurement results of the sensor are not affected by liquid velocity, sensor′s inclination angle, and temperature change. The liquid level, pressure and temperature sensitivity of the sensor reach 35. 16 pm/ cm, 359. 46 pm/ kPa and 10. 07 pm/ ℃ , respectively. The maximum relative error is 5. 6% .