Accurate fundamental-mode (FM) reflectivity measurements of FM weak reflection large-mode-area (LMA) FBGs constitute a challenging problem owing to high-order modes (HMs). In this paper, we propose a novel measurement method that uses scale gratings fabricated in a single-mode fiber. The weak reflectivity of the measured FBGs was achieved by comparing the peak reflection resonance with that of the scale gratings. The measured minimum reflectivity of the scale grating was 1.3%, giving the measurement accuracy. The accuracy can be improved further by increasing the number of reflectivity scales. A Fabry-Perot-interferometer-based scale grating was proposed, designed, and fabricated using a chirped phase mask to achieve soaring scale numbers. The minimum reflectivity of the scales decreased by 0.37%. Several LMA double-cladding FBGs with weak reflectivity were measured. The results show that this novel measurement method is convenient and efficient, does not depend on the transmission spectrum of the grating, and can circumvent the influence of HM. It is anticipated that weak reflectivity can be measured more accurately by improving grating fabrication technology.