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A Raman sensor based on a cavity-enhanced Ag nanoparticle (AgNP) decorated tapered fiber is proposed. Its Raman enhancements are mainly caused by the localized surface plasmon resonance effect of AgNPs decorated on the tapered optical fiber surface and the further reflective laser excitation induced by the capillary-based reflective cavity. We theoretically investigate the backward Stokes power conversion efficiency and cavity enhancement factor of the sensor. The calculated relationship between the cavity enhancement factor ξ and distance L from the tip to reflective rod is also discussed. Subsequently, the proving experiments were carried out for a tapered fiber surface-enhanced Raman scattering (SERS) probe and cavity-enhanced metal decorated tapered fiber Raman sensors. The analytical enhancement factor is 5.51×10^4 for the tapered fiber SERS probe. Moreover, the predicted curves of the theoretical model are close to the experimental values. This Letter provides a possible way to rigorously quantify the complete coupling efficiency for tapered fiber SERS probes, as well as cavity enhancement factors of cavity-enhanced Raman sensors.