Provided is an apparatus and method for reconstructing a computed tomography (CT) image using a color channel of a graphic processing unit (GPU) that reconstructs a three-dimensional (3D) image using a projection image obtained from a CT device. According to an embodiment of the present invention, an apparatus for reconstructing a CT image may include a tomography unit to acquire a plurality of projection images, a filter application unit to load the plurality of projection images on a texture memory having a color channel, and filter the plurality of projection images, and a back-projection application unit to apply a back-projection scheme to the plurality of projection images loaded on the texture memory having a color channel.