This paper analyzes the measuring results of a diffraction efficiency measuring instrument for concave gratings to improve the measuring accuracy of relative diffractive efficiency. According to the testing theory of diffraction efficiency of concave gratings, the effects of the spectral broadening of exit beam for the concave grating, section changing of diffraction beam, control of light source radiation luminance and the synchronization precision of testing wavelength on the measuring accuracy were analyzed, and several kinds of necessary equations were given. Based on regression analysis and a large number of experiment data, an optimization equation was established and programmed into a testing programming to correct automatically the test results. Experiments indicate that the tested results are correct and more precise, and the difference between optimization value and theoretical one is less than　±2.5%, which improves the testing precision effectively. This method is simpler operation, not to change the components of the instrument, and can satisfy the test requirements in strong real-time and higher precision.