The invention discloses a method for calculating the signal-to-noise ratio (SNR) in parallel acquisition image reconstruction, comprising: determining a reconstruction expression for a linear operation of the image reconstruction; determining a weighted coefficient according to the reconstruction expression; calculating the SNR according to the weighted coefficient and the raw data. The SNR not only is relevant to the geometric shape and position of the coils, but also is influenced by the reconstruction method and the sampling mode. The SNR is calculated based on contribution of the raw data at positions in the reading direction from all the phase-coding lines in all acquisition channels. It reflects more precisely the loss of the SNR in the parallel acquisition image reconstruction, especially the changes in the SNR caused by the number of the reference lines combined during the reconstruction.