1. An apparatus for acquiring a multi-energy X-ray (MEX) image, the apparatus comprising: an X-ray source which generates X-rays and irradiates the X-rays onto an object; an energy identification detector which acquires the MEX image being generated when the X-rays penetrate the object; and a MEX processor which generates an optimal MEX parameter based on a characteristic of the object, and controls at least one of the X-ray source and the energy identification detector based on the generated optimal MEX parameter.