To improve the measurement accuracy of a plane grating diffraction efficiency measuring instrument, the affect factors on the diffraction efficiency measurement of a plane gating were researched. According to the measuring principle of relative diffraction efficiency of the plane grating, the concept of beam cross-section factor k(θ) of plane grating was proposed and the correspondent expression was developed. Then, the factors affecting the measurement accuracy of diffraction efficiency, the spectral width and the beam cross-section factor were analyzed, and two independent variables for the regression were defined by combination with the measurement value of diffraction efficiency. Regarding the nonlinear relation between the theoretical value of dependent variable and the independent variable defined, the complete quadratic regression analysis was applied as the methodology to establish a revised equation to improve the measurement accuracy of the diffraction efficiency. The results show that the variation between the revised and the theoretical values is less than 0.5%, which means the measurement accuracy of diffraction efficiency has been improved and the performance of the instrument is enhanced significantly.