The invention discloses a digital filtering method for image zooming-out, comprising that: a first line of data in the current source image data is carried out the horizontal filtering treatment according to the horizontal widths of the source image and a target image, the result data is saved in a cache unit; the next line of the data in the current source image data is carried out the horizontal filtering treatment according to the horizontal widths of the source image and the target image, the result data is correspondingly added with the data in the cache unit one by one, the result data after the adding is covered on the original data in the cache unit; the process is carried out like this till the last line of the data in the current source image; and the data in the cache unit is output as the data of the target image of the data of the current source image. The technical proposal of the invention does not need to consume a lot of storage media.