The crystal structures of actinium dihydride (AcH2) in a wide pressure range of 0-200 GPa have been investigated by using an unbiased structure searching method coupling with first-principles density functional calculations. A series of pressure-induced phase transitions are predicted for AcH2, as Fm-3m -> P4(2)/mmc -> Imma -> P6/mmm, and the calculated pressures of the phase transitions are 12, 27, and 68 GPa. Under pressure, the coordination number of H atom increases and the nature of chemical bonds changes. The electronic band structures show that all of the competitive phases are metallic. The pressure-induced charge transfer drives the phase transition. Phonon calculations show the competitive phases are thermodynamically stable within their favored pressure range. Our results elucidate the phase transition mechanisms of AcH2 under pressure, and have major implications for the high-pressure behaviors of actinide and rare-earth metal dihydrides. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.