1. An image registration method, comprising steps of: obtaining a spectrogram of infrared images; detecting a first plurality of feature points on a target image being at least one of a first single image and a first series image of the spectrogram, and a second plurality of feature points on a source image being at least one of a second single image and a second series image of the spectrogram, wherein the first plurality of feature points has a first plurality of cross-points of the vessel middle lines and the second plurality of feature points has a second plurality of cross-points of the vessel middle lines; searching a plurality of corresponding point pairs to determine corresponding relationships between the first plurality of feature points on the target image and the second plurality of feature points on the source image, wherein the corresponding point pair includes a pair of feature points; and correcting locations of the plurality of corresponding point pairs to obtain a registered source image.