1. A method for calculating a signal-to-noise ratio of a parallel-acquired image of an object in a parallel acquisition image reconstruction, comprising: acquiring the parallel-acquired image by a MRI device; determining a reconstruction expression for a linear operation of the image reconstruction, ρ=Mr, wherein ρ is a reconstructed image of the parallel-acquired image, M is the reconstruction expression, and r is raw data of the parallel-acquired image by an image processing unit; determining a weighted coefficient of the image reconstruction according to the reconstruction expression by the image processing unit; and calculating the signal-to-noise ratio of the parallel-acquired image according to the weighted coefficient and the raw data by the image processing unit, wherein the raw data comprises a uniform under-sampling data in K-space, or wherein the raw data comprises a mixed sampling data by combining the uniform under-sampling data and low frequency full-sampling data in the K-space.