As the double-rod structure in ruling system of a large diffraction grating ruling machine can not meet its accuracy requirement,this paper designs a single-rod structure to substitute it.The reasons of bending error of indexing direction of quarts guide rail in the grating ruling machine were discussed and the method to reduce the error was analyzed.Meanwhile the rod structure and the stress on a saddle slider were analyzed.Then,based on the theory of bending deformation of material mechanics,the model of bending error in indexing direction of quartz guide rail was established.On the basis of this model,the bending errors of quartz guide rails on the double-rod structure and the single-rod structure were simulated.Finally,a frequency laser interferometer was used to measure the two feature points on quartz guide rail.Experimental results indicate that the bendings at the measuring points of quartz guide rail are reduced from 50.36 nm to less than 10 nm by using the single-rod struture.It concludes that the single-rod structure basically meets the requirement of large diffraction grating ruling engine for the measuring precision of 5—10 nm.