In the experiment of high-precision spectral calibration, the results of spectral calibration, when using the tilt diffuse reflector as dodging device, are different from vertical exposure. Through analysing the process of energy transfer, we find that the radiant flux on the entrance slit is uneven when using the tilt diffuse reflector, which causes the wavelength difference. We use the double grating monochromator as the instance, and put all of the radiant flux of different wavelengths on the entrance slit. And then we can know that wavelength difference is -0.0283 nm, when using the 45°tilt diffuse reflector to wavelength calibration. Finally, making the mercury as light source, by scanning the 296.7283 nm and 365.0157 nm wavelengths with two methods of using the diffuse transmittance plate to vertical irradiation and using the 45° tilt diffuse reflector plate irradiation, the wavelength difference are -0.029 nm and -0.0286 nm. The results of the experiment can certify that the tilt reflect diffuser is the main factor to cause the wavelength difference in the spectral calibration experiment.