The invention discloses a method for utilizing multi-energy X-ray composite projection to digitally synthesize images. The method sequentially includes the following steps: (A) projected image signal acquisition: n X rays with different energies are adopted to respectively project s image signals on m different spatial positions, wherein n is larger than or equal to 1, m is larger than or equal to 1, and n multiplied by m is s; (B) projected image preprocessing: analog projected image signals are first amplified and converted into digital image signals by a signal amplifier and an analog-to-digital converter; the digital image signals are then inputted into a digital image preprocessor; and the digital image preprocessor compensates the sensitivity of each received pixel unit; (C) projected digital image synthesis: by utilizing digital image processing, a digital image synthesizer synchronizes the amended digital images into a series of synchronized digital images; (D) synchronized digital images post-processing: the requirements of diagnosis can be met; and (E) the output of the synchronized digital images: the processed images are outputted to a terminal device.