The four forms of beamexpander with a two-mirror system are compared in this paper.The conclusion has beengiven,which explains a beam expaader with two parabolic mirrors can eliminate sphericalaberration,coma and astigmatism at any ratio of shade or magnification,and therelationship between coefficient of distortion and ratio of shade is given too. Therelationship between coefficient of field curvature and ratio of shade are got throughlight traces. The different light rays are drawn apart when the ratio of shade is positiveor negative and so do their off-axis forms.