1. An image compensation method, comprising: calculating a position range for reconstructing a complete image by using a center of a first scanning as a symmetry point; calculating an overlapping position between two adjacent scannings according to the position range for reconstructing the complete image; obtaining two images at the overlapping position of the two adjacent scannings, and calculating mutual information of the two images by using a three dimensional non-rigid registration method; and acquiring a corresponding transformational matrix when the mutual information reaches a threshold, and compensating one of the two images which needs to be compensated by using the transformational matrix.