The invention discloses an image processing method used for brain MRI image classification. The Image processing method includes the steps of: obtaining a first-type MRI image set and a second-type MRI image set; extracting similarity information of the first-type MRI image set and the second-type MRI image set; using a Nystrom algorithm and a projection algorithm to calculate Euclid coordinates of individuals in the first-type MRI image set and the second-type MRI image set; using the similarity information and the Euclid coordinates to perform classification training on the first-type MRI image set and the second-type MRI image set, thereby obtaining a judgment criterion; extracting image features of an MRI image set to be classified, and performing image classification according to the judgment criterion. The invention also discloses a device based on the abovementioned method. The image processing method used for brain MRI image classification adopts the Nystron algorithm to approximate a similarity matrix, thereby greatly reducing the calculated amount.