1. An MRI water/fat image identification method, comprising: obtaining, via a processor, a water/fat image pair calculated by a Dixon method, and marking the pair as a first image and a second image; calculating, via the processor, grayscale histograms for the first image and the second image, respectively, to obtain a first grayscale histogram and second grayscale histogram; subtracting, via the processor, the second grayscale histogram from the first grayscale histogram to obtain a water/fat relationship graph; and searching, via the processor, a relatively highest peak and relatively lowest trough in the water/fat relationship graph, and judging a positional relationship of the relatively highest peak and the lowest trough in the water/fat relationship graph; and determining, via the processor, the first image as being a water image and the second image as being a fat image if the relatively highest peak is located behind the relatively lowest trough; and determining, via the processor, the second image as being a water image and the first image as being a fat image if the relatively highest peak is not located behind the relatively lowest trough.