  Large orbital angular momentum can be generated in non-central heavy-ion
collisions, and part of it is expected to be converted into final particle's
polarization due to the spin-orbit coupling. Within the framework of A
Multi-Phase Transport (AMPT) model, we studied the vorticity-induced
polarization of $\Lambda$ hyperons at the midrapidity region $|\eta|<1$ in
Au-Au collisions at energies $\sqrt{s_{NN}}=7.7\sim200$ GeV. Our results show
that the global polarization decreases with the collisional energies and is
consistent with the recent STAR measurements. This behavior can be understood
by less asymmetry of participant matter in the midrapidity region due to faster
expansion of fireball at higher energies. As another evidence, we discuss how
much the angular momentum is deposited in different rapidity region. The result
supports our asymmetry argument.
