The present invention discloses an image capturing device and an image synthesis method thereof. The image capturing device comprises an image capturing module, an exposure module, and a processing module. The image capturing module captures a high bit-depth scene image corresponding to a scene. The exposure module meters the light of the scene to generate a plurality of histograms. The processing module calculates a plurality of discrete values of different exposure conditions of the high bit-depth scene image based on the plurality of histograms, processes the high bit-depth scene image to generate at least two low bit-depth temporary images in different brightness conditions based on at least two highest discrete values, and synthesizes the at least two low bit-depth temporary images in different brightness conditions to generate a high dynamic range image.