In some case,the axial distance from the sample to the lens is long in laser particle size measurement,so vignette appears because the lens aperture is limited,which reduces the measure precision and accuracy.In order to solve the problem,the gray system theory is presented to check the optical signal data to find the vignette point.And then,the signals after the vignette point are predicted by grey-prediction model.By this way,the vignette data are amended.Experiments on a particle plate were carried out.The particle plate was placed at several positions in front of the lens.When the data were modified,a no-model algorithm was used to invert the particle size distribution.The results show the gray-prediction method is valid.The purposed means can be used to enhance the precision and accuracy in some measurement,where the axial distance from the sample to the lens is long or the covered area of the sample is large.