TOPS (Terrain Observation by Progressive Scans) mode can cover large azimuthal scenes in a short time through beam scanning. Aiming at airborne FMCW SAR system, the imaging issues of airborne TOPS mode are studied. By analyzing the characteristics of echo data in this mode, the original frequency scaling algorithm (FSA) has fan-shaped deformation caused by the inconsistency of azimuthal sample interval at different ranges during azimuth scaling, thus an improved frequency scaling algorithm which exploit baseband azimuth scaling method as azimuth process is proposed in this paper to solve the problem. The validity of the proposed algorithm is verified by simulation and equivalent measured data.