Arnold transformation is applied widely in digital image encryption because of its periodicity. But if its periodicity is used in the course of image decryption, it will waste lots of time. So this paper offers a new anti- Arnold transformation algorithm. The algorithm gets anti-Arnold transformation by solving equation groups. The application of Arnold transformation from square image to rectangle image is generalized and its application on image encryption expanded. At the same time, the anti-Arnold transformation from 2-dimension to m-dimension is also generalized.