In order to get precise testing results from the instrument used for measuring the diffraction efficiency of plane grating, the analysis has been done on the instrument. Through analyzing the main factors that spectral broadening of exit beam and cross-section change of diffracted beam, which influence the testing results during measurement, the relation between wavelength and groove density of them is given. Based on a large number of experimental data, multiple linear regression analysis is used to establish regression equation based on two primary factors which influence the testing result during the testing process, and the equation is proved to be exact and credible not only in theory but also in experiment. Furthermore, the equation has been programmed in the testing programming, and it can correct the testing results automatically after finishing the test. By the results, the testing results which have been corrected are closer to the theoretical value, and the difference is less than ±2%, which prove that the method can be used to measure the diffraction efficiency of the plane grating. This method is strongly real-time, and any change is not necessary to the components of the instrument, which can satisfy the requirements of simple operation, testing speediness and preciseness.