High-reflectivity fiber Bragg grating (FBG) is successfully inscribed on the 20/400 μm large-mode-area double-clad fiber (LMA-DCF) by the phase-mask method, and the relevant technical requirements and difficulty of the inscription technology are analyzed. Considering the possibility of existence of the high-order mode in LMA-DCF, the mode characteristics are theoretically simulated by altering the relative displacement between the fiber core of SMF and LMA-DCF. The grating reflection spectra of different relative displacements are experimentally measured and the corresponding light spots are obtained. Besides, the frequently-used measuring methods of spectrum of LMA-DCF Bragg grating are compared and their respective characteristics are analyzed. Research results will provide theoretical and experimental basis for the application of LMA-DCF in high-power laser systems.