1. A fast GeneRalized Autocalibrating Partially Parallel Acquisition (GRAPPA) image reconstruction method for magnetic resonance imaging, comprising the steps of: (i) expressing s fitting calculation of a channel in the frequency domain as a convolution of channels and transforming the convolution into the image domain; (ii) converting a channel merging calculation in the image domain into a linear calculation of images of individual channels and sensitivities of the coils; and (iii) merging the linear calculations in steps (i) and (ii) as a weighting matrix.