This paper provides a systematic introduction to the principle of FP cavity fiber optic sensors based on thin film technology and reviews the applications and development trends of this sensor in various measurement fields. Fiber sensors possess characteristics such as compact structure, simplicity, electromagnetic interference resistance, and reusability, making them widely applicable in various practical engineering applications. Traditional fiber sensors based on different microstructures solely rely on the thermal. The vernier-effect-based sensitivity enhancement of two kinds of sensing units consisting of dual fiber Fabry-Pérot (FP) cavities in the Optical Frequency Domain Reflectometry (OFDR) is analyzed in this paper. Theoretical analysis reveals that significant differences exist in the sensitivity.