A method and device of fingerprint image generation for saving memory. The method includes generating a first fingerprint image of an original data size according to a plurality of first analog sensing signals which are read from a fingerprint sensor array before an exposure period ends. Then the first fingerprint image represented by a first data size which is equivalent to or smaller than the original data size is stored. a second fingerprint image of the original data size is generated after generating the first fingerprint image of the original data size according to a plurality of second analog sensing signals which are read from the fingerprint sensor array during the exposure period. The second fingerprint image represented by a compressed data size smaller than the original data size is then stored.