The effects of alternating current poling (ACP) at 80 °C on electrical properties of -oriented 0.72 Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 (PMN-28PT) single crystals (SCs) have been investigated. The square-wave ACP SCs poled at high voltage (HV, 5 kVrms/cm) occasionally showed large fluctuations and low opposite values of piezoelectric coefficient (d33 = ± 1370 pC/N) in one plate. This revealed spurious-mode vibrations (SMV) of impedance spectrum. However, after depolarizing and repolarizing the sample with a sine-wave ACP at low voltage (LV, 3.5 kVrms/cm), the d33 enhanced to be 1720 pC/N (+26%) and did not exhibit large fluctuation or opposite values in one plate any more. The impedance spectrum became clean and the abnormal SMV disappeared. We proposed four possible mechanisms of the SMV, and speculate that the main cause maybe by macro-scale sub-domain structure and/or phase change in the main domain structure and/or phase in the SC plate due to the specific poling conditions not eternal mechanical damage of PMN-PT SCs. This study will be useful to realize a high d33 and improve other properties of PMN-PT ACP SC ultrasonic transducers without any SMV for high-frequency medical imaging equipment.