The problem of displacement of center of rotation is discussed in CT image reconstruction by convolution back projection. Basing on ideal convolution back projection, a convolution back projection with a shift center of rotation has been derived for correcting images that acquired with a divergence center of rotation using the fan beam geometry with an angle equaled detector. In order to solve parameter of divergence axis of rotation, a single pixel phantom has been introduced. Software simulations are performed to certify correcting method's feasibility, as a result, eliminate the tail off artifacts and improve the resolution of CT image.