To immune the influence of compositions and refractive indexes (RIs) of working media on the accuracy of liquid level measurement, a reflective fiber-optic liquid level sensor is prepared. The sensor is composed of a side-emitting plastic optical fiber with a low cladding RI, reflector, optical fiber support rod, and air sealing device. First, the effect of the RI of the optical fiber cladding on the liquid level measurement of different working media and the same working medium with different RIs is studied theoretically and experimentally. Then, in order to improve the sensitivity of the sensor, the effects of fiber spiral diameter and pitch on the sensitivity of the sensor are studied. Finally, the effects of liquid level change velocity and working medium temperature on the response characteristics of the sensor are studied. The results show that there is a linear relationship between the output of the sensor and the liquid level when the RI of the working medium is larger than the RI of the optical fiber cladding, the change velocity of the liquid level is 10--100 cm/min, and the temperature range of the working medium is 10--70 ℃. Moreover, the measurement results are not affected by the composition, RI and liquid level change velocity of the working medium, the sensitivity of the sensor can reach 0.0101 cm ^-1, and the maximum relative error is less than 6.85%.