Abstract: The criterion of optimizing the fringing magnetic field is discussed, and ananalytical-numerical comprehensive method for realizing the optimization is intro-duced in the paper. The method mentioned above consists of two parts, the ana-lytical part calculates the field of the shims, which corrects the fringingmagnetic field by using uniform magnetizing method; the numerical partfulfils the whole calculation of the field distribution by solving the equation ofmagnetic vector potential A within the region covered by arbitrary triangularmeshes with the aid of finite difference method and successive over relaxationmethod. On the basis of the method, the optimization of the fringing magneticfield for a large-scale electromagnetic isotope separator is finished.