To detect liquid levels accurately, a liquid-level sensor is fabricated by coupling fiber Bragg grating (FBG) and an elastic diaphragm. The sensor is composed of two FBGs (FBG_1 and FBG_2), an elastic diaphragm, an elastic rubber sealing gasket, a hollow tube (an air duct), and an FBG fixing device. The FBG fixing device is composed of polymethyl methacrylate. FBG_1 and FBG_2 are fixed to the surfaces of elastic diaphragms. FBG_1 is used to monitor the liquid level and temperature. FBG_2 is encapsulated in a stainless-steel capillary tube to detect the temperature and mitigate the adverse effect of liquid-phase temperature changes on liquid-level measurement. The effect of the elastic-diaphragm thickness on liquid-level measurement is experimentally investigated to improve the linearity of the sensor’s liquid-level measuring capabilities. Additionally, we investigate the effect of the liquid-level change rate and external vibration on the liquid-level measurement accuracy. The results show that under an elastic-diaphragm thickness of 2 mm, the sensor’s linear range for detecting the liquid level is 0-380 cm, with a sensitivity of (19.65±6.3) pm/cm. The sensor can accurately monitor the liquid level within a liquid-level change rate of 0-100 cm/min and an external vibration intensity of 0-5.27 mm/s, with a maximum detection error of 7.2%.