An optical module positioning method for an image processing device is provided. The image processing device includes a scanning platform and an optical module. The scanning platform includes a first region, a second region, a third region and a fourth region. The fourth region is arranged between the first region and the second region. The first region is arranged between the third region and the fourth region. The optical module positioning method includes the following steps. Firstly, the optical module is driven to perform a scanning operation to acquire a scanned image while the optical module is not moved. Then, by judging the color values of two ends of the scanned image, the region where the optical module is stayed can be recognized according to the associated color values.