In order to make the process of CSAR (circular synthetic aperture radar) imaging more efficient, a fast time-domain CSAR imaging method was presented. The proposed method was based on the circular aperture of the CSAR model, and the full aperture was divided into several sub-apertures; those sub-apertures were processed by the fast factorized back projection algorithm to get their corresponding sub-images; the final focused image was obtained by the coherent interpolation of the sub-images from the polar coordinate to the global coordinate system. Some key problems in the algorithm realization, such as coordinate transformation, error control and computation efficiency, were analyzed in detail. Experiments results based on the simulated data and real data were given to prove the validity of the proposed method. The proposed method has the advantages of improving imaging extension and lowering computational complexity.