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The magnet system is an important part in magnetic resonance imaging (MRI) equipment, the performance of which directly affects the quality of the imaging. Improving the homogeneity of the magnetic field is an effective means to increase the signal-to-noise ratio (SNR). A high SNR ensures a good imaging. This paper mainly investigates the optimal design and homogenizing field analysis of main magnet for ferrite yoke flat-board type in superconducting MRI using the finite element method (FEM). In order to get a high homogeneous magnetic field, the pole piece is added on the magnetic pole. We use electric pure iron as the material of the pole piece which has low coercive force, high permeability, and high saturation flux density. In order to increase the calculation speed, three-dimensional analysis is replaced by two-dimensional analysis while keeping the precision adequate. Through the optimizing algorithm, we can obtain an ideal pole piece dimension of the magnet. The distribution of the magnetic field was simulated and tested experimentally. By comparing the numerical calculation results with the experimental results, we find that a superconducting magnet MRI having homogeneity in volume of interest less than 600 ppm can be designed by optimizing the dimension of the pole piece using the proposed method without shimming.