Complex hydrides are potential candidates for the solid electrolyte of all-solid-state batteries owing to their high ionic conductivities, in which icosahedral anion reorientational motion plays an essential role in high cation diffusion. Herein, we report molecular dynamics (MD) simulations based on a refined force field and first-principles calculations of the two complex hydride systems Li2B12H12 and LiCB11H12 to investigate their structures, order–disorder phase-transition behavior, anion reorientational motion, and cation conductivities. For both systems, force-field-based MD successfully reproduced the structural and dynamical behavior reported in experiments. Remarkably, it showed an entropy-driven order–disorder phase transition associated with high anion reorientational motion. Furthermore, we obtained comparative insights into the cation around the anion, cation site occupancy in the interstitial space provided by anions, cation diffusion route, role of cation vacancies, anion reorientation, and effect of cation–cation correlation on cation diffusion. We also determined the factors that activate anion reorientational motion leading to a low to high conductivity phase transition. These findings are of fundamental importance in fast ion-conducting solids to diminish the transition temperature for practical applications.