Fiber Bragg Grating (FBG) sensing technology serves as a crucial approach for monitoring groundwater seepage velocity in boreholes, where the accurate determination of flow field distortion coefficient is a key factor for its precise measurements. This paper based on a single-hole velocity correction model for FBG seepage monitoring, conducts experimental investigations on the distortion coefficient in FBG monitoring seepage velocity, aiming to explore the variation patterns of the distortion coefficients in underground seepage through geotechnical media under different seepage conditions. Firstly, a distortion correction model for FBG monitoring seepage velocity was derived based on the principle of single-borehole velocity correction, and the three main influencing factors of the distortion coefficient, including seepage velocity, filter pipe diameter and type, were determined. Subsequently, an in-situ groundwater seepage monitoring device was designed and fabricated. Combined with the self-made FBG seepage monitoring device, followed by experimental investigations on FBG-based seepage velocity monitoring under various borehole layout forms. The influence of different seepage velocity, filter tube diameters and types on the distortion coefficient were studied. The experimental results showed that by applying the distortion coefficient to correct the borehole-measured seepage velocities obtained from the FBG seepage monitoring device under various borehole layout forms, the actual seepage velocity within the aquifer can be determined with improved accuracy. When the seepage velocity exceeded 0.05 mm/s, the distortion coefficient exhibited a well-defined pattern: the distortion coefficient increased with higher seepage velocity, decreased with larger filter tube diameter, and was greater for bridge filter tubes compared to circular filter tubes under the same layout. The influence of the seepage velocity (the permeability coefficient of aquifer), filter tube diameter, and type on the distortion coefficient varied, with the order of sensitivity being the seepage velocity (the permeability coefficient of aquifer) > filter tube type > filter tube diameter. The findings of this study provide valuable reference for the practical application of FBG technology in subsurface seepage monitoring.