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In inverse synthetic aperture radar (ISAR) imaging of targets with complex motions such as fluctuating ships with oceanic waves and high maneuvering airplanes, the azimuth echo signals can be modeled as cubic phase signals (CPSs). In this letter, a new ISAR imaging algorithm based on the keystone time-chirp rate distribution (KTCRD) is proposed for the targets with complex motions. Compared with the recently published algorithms for the CPSs, the KTCRD can estimate the parameters of multicomponent CPSs without searching procedures and can acquire high antinoise performance with a relatively low computational load. With the estimated motion parameters, high-quality ISAR images can be obtained. Several simulation examples on the synthetic model are shown to validate the effectiveness of the new algorithm presented in this letter.