Abstract: The Circular Synthetic Aperture Radar (CSAR) is a special spotlight SAR (synthetic aperture radar) with an all-directional high spatial resolution. The feasibility of its space-borne implementation by a space-borne platform is analyzed based on a dynamical method. A trajectory model with the constraint of CSAR's operating conditions is built. And an active control force on the space-borne platform to keep its trajectory is determined by analyzing the motion and the load of the platform by a basing-point method. Results show that for any latitude area, the magnitude and the direction of the force needed vary too frequently to be actualized. For the north and south pole areas, to keep the platform moving along a circular trajectory with a certain angular velocity, the magnitude and the direction of the active control force needed vary slightly and are easy to be actualized.