Fiber optical reflectometer (FOR) is a novel technique for dense droplet detection that combines conventional phase detection with non-invasive velocity measurement capabilities. In this study, the mechanism of FOR technique and its sensitivity to droplet trajectory are investigated by wave optics simulation and experiment. This paper first describes the simulation method and analyzes the dynamic response of probe during the droplet piercing process. Next, an exploration is conducted on the effective field of view for the probe and the detectable interface inclination. Afterwards, by precisely controlling the droplet size, eccentricity, and trajectory, the simulation reveals the sensitivity of FOR technique to droplet parameters and quantifies the accuracy of measurement on droplet velocity, which provides specific guidance for the design and application of FOR technique. Finally, in the static experiment, the measurement error of the droplet axial velocity is controlled within 13 %, which verifies the reliability of the simulation results.