The formula of △T anomaly from the 2.5 D polygonal body has been deduced and an optimum procedure that computes the magnetic model is described in detail. As a result of the application of SVD to the solution of Generalized Matrix Inversion, the problem related to the compromise between speed and stability of the convergence has been solved. In this way, the best model in the sense of the least squares can be obtained. With two theoretical models and the data obtained in Xianiu area, this paper illustrates the effecti veness of the method.