Radar imaging technology has attracted increasing attention in the field of deep space exploration due to its fast， non-destructive， and high-resolution characteristics. To address the issue of low computational efficiency in synthetic aperture radar （SAR） 3D imaging， a fast factorized back-projection （FFBP） 3D imaging algorithm suitable for slow flyby observation modes is proposed leveraging the weak gravity and rapid spin characteristics of small solar system bodies. Initially， the equivalent motion model under slow flyby mode is analyzed， extending the imaging domain from a 2D polar coordinate system to a 3D spherical coordinate system. An in-depth analysis of aperture division and image fusion issues within the 3D FFBP algorithm is conducted， deriving rules for 2D sub-aperture division and recursive image fusion methods， along with a detailed implementation process. Finally， the effectiveness of the algorithm is validated through numerical simulations and measured data. Experimental results show that the proposed imaging algorithm significantly enhances computational efficiency. Compared to the back-projection （BP） algorithm， it can achieve a speedup ratio of 30—50 times while obtaining imaging performance comparable to the classical BP algorithm.