Abstract: Using a geometrical optical method, a model for analytic calculation about ellipticity error effect in neutron penumbral imaging is established. An analytic formula of neutron absorbing length is derived from the model, and a general analytical expression for neutron penumbral imaging process is achieved. Based on this model, a numerical simulation of image aberration caused by ellipticity error is realized. The point spread functions under various ellipticity errors and their effects on a round object’s reconstruction are obtained. A semi-quantitative result shows that the effect on imaging quality can be omitted when the ellipticity error is less than 0.01. Compared with the Monte-Carlo method, the analytic model is more effective in computation and easy to apply to other similar code i