Large aperture multilayer dielectric grating (MDG) is one of the key elements of high-power laser systems. In order to meet the requirements for large aperture MLG in a high-power laser system, a diffraction efficiency measurement system is developed for characterizing the diffraction efficiency of large MDG at 1064 nm and Littrow mounting. Through analyzinge the main factors such as detector signal-to-noise ratio and operating staff, which influence the testing results during measurement, their difference is obtained to be less than ±1%, which proves that the method can be used to measure the diffraction efficiency of MDG. Based on a large number of experimental data, the optical characterization of MDG in a negative first-order Littrow configuration could provide some information about grating microstructure. So the diffraction efficiency measurements have a number of important applications in the fabrication process, as a nondestructive grating inspection tool, such as determining the suitable photoresist grating mask which meet the ion beam etching requirements, on line end-point detection during ion beam etching, and judging whether another etching is needed. Based on above techniques, a number of MDGs have been achieved, each of which has mean diffraction efficiency greater than 95% line density 1740 lines/mm, and aperture up to 430 mm×350 mm. The results show that this method can satisfy the requirements for simple operation, testing speediness and preciseness, in which any inspection is not necessary for the MDG microstructure paramters.