ISAR imaging for a marine moving target faces a series of challenges in the airborne radar system, especially the strong clutter interference with Doppler frequency spreading. Space-time adaptive processing (STAP) has advantage of adaptive clutter suppression, however, the conventional STAP is not applicable for the ISAR system due to some significant differences, To tackle this problem, an efficient ISAR imaging method based on sliding window STAP is proposed for the airborne radar system in this paper. In the sliding window STAP, the whole CPI is divided into a series of coherent processing sub-intervals (CPSIs). Those CPSIs are generated with sliding window technique and they have identical length. In each CPSI, STAP algorithm is adopted to suppress the clutter. After that, the target signal is enhanced but the range migration still exists. Meanwhile, the Doppler frequency differences with respect to the azimuth dimension is still maintained, which guarantees the further ISAR imaging by Range-Doppler (RD) algorithm. The proposed method can improve the ISAR imaging performance with low signal-to-noise ratio (SNR). The simulation experiments are carried out to verify the effectiveness of the proposed method.