Based on the analysis of large-mode-area (LMA) double-clad fiber′s mode properties and optical power distribution in the measurement setup, reflection and transmission spectra of LMA fiber gratings are numerically simulated by using coupled-wave theory and transfer-matrix approach. Theoretical results indicate that the spectral shape depends on the power distribution between modes, and the reflectivity of LMA fiber grating can be calculated from the transmission spectrum of the fundamental mode. LMA fiber grating with the reflectivity of more than 99.7% has been fabricated in 20/400 μm double-clad fiber by phase mask method, the reflection and transmission spectra under different conditions are measured, and the experimental results agree well with the theoretical analysis.