According to fundamental principle of optical Fabry-Pérot cavity, the work principle and design project of optical fiber liquid-level sensor based on extrinsic and sensitive Fabry-Pérot cavity are analyzed. The experimental results indicate that the sensor possesses good characteristics of several periodicities output and partial linearity. But output intensity of the F-P cavity attenuates gradually with increase of cavity length, and the same length linear work intervals in different segments of output characteristic curve of the sensor correspond with different measurement ranges and sensitivities. The sensor can satisfy requirements for measuring different liquid-level by changing dimension of sensitive module in the sensing head. Precision of ±0.5 mm and minimal resolving power of 0.3 mm over a full range of 20 m are obtained. The sensor can be applied to measure liquid-level of flammable and explosive liquid accurately and has strong practicability.