During the thinning process of plate-shaped optical parts (PSOP), the release of internal stress would cause the deformation of ultra-thin PSOP, which deteriorates the processed surface figure. The stress-release-induced deformation is hard to be predicted and controlled due to the difficulty in measuring the tiny internal stress in ultra-thin PSOP. In this paper, an analytical model is established to depict the variation of internal stress and deformation during the thinning process. It can be used to calculate the initial internal stress distribution along the thickness according to the deformation and the residual thickness of the sample. Meanwhile, the model can predict the residual stress distribution and deformation in the whole thinning process. The prediction results obtained from the proposed model agree well with the experimental results, and the prediction error is less than 13%. The presented model has great significance for the analysis of the tiny internal stress and then guide the process of making ultra-thin PSOP.