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The impurities of various harmonic orders in region of MRI magnetic field will depress the qualities of the image. Based on the direct proportional relationship of FID signal of MRI system and magnetic field homogeneity, an efficient optimization algorithm of active shimming is presented to correct the inhomogeneous field. FID intensity is used as feedback information to calculate the shim currents with adaptive theory. It makes the calculation process independent of the coils hardware parameters and the correction function of the coils be put forth furthest. The experiment results show that the optimal shim currents can be quickly worked out in less than 3 minutes. Magnetic field homogeneity and qualities of the image are evidently improved by the novel method.