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Owing to the unknown high-order unstable motion, it is difficult to realise the motion compensation, that is, envelope alignment and phase auto-focusing, for inverse synthetic aperture radar (ISAR) manoeuvring targets. By modelling the envelope shifting and phase modulation into two different high-order polynomial functions against observation time, a novel parametric ISAR motion compensation method is proposed based on polynomial coefficients estimation via particle swarm optimiser (PSO). The average range profile energy and the image contrast are chosen as the fitness functions for envelope alignment and phase auto-focusing, respectively. Furthermore, the polynomial order of phase auto-focusing is chosen higher than that of envelope alignment to meet the accuracy need of phase compensation. Besides, in order to speed up the convergence and ensure that the estimated parameters converge to the global optimisation, a least-square fitting pre-processing is also proposed to determine the target motion order and initialise the best particle at first. Finally, the results based on both numerical experiments and real data are all provided to demonstrate the effectiveness of the proposed PSO-based parametric methods.