A device for synthesizing a high dynamic range image based on per-pixel exposure mapping includes an image-capturing module, an image-processing module, and a recursive control module. The image-capturing module serves to obtain a plurality of source images each having a unique exposure value. The image-processing module uses an exposure-value modulating means to perform exposure synthesis to pixels on an identical location of two of the source images that have similar exposure values according to exposure values of the pixels, so as to form synthesis images. The recursive control module sends the synthesis images back to the image-processing module for reprocessing until the image-processing module generates a single synthesis image.