1. An MRI water/fat image identification method, comprising: obtaining a water/fat image pair calculated by a Dixon method, and marking the pair as a first image and a second image; calculating grayscale histograms for the first image and the second image, respectively, to obtain a first grayscale histogram and second grayscale histogram; subtracting the second grayscale histogram from the first grayscale histogram to obtain a water/fat relationship graph; and searching for 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 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 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.