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Within the framework of Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model, we extract the global spin polarization of Λ hyperon in 108Ag+108Ag and 197 Au+197Au collisions at sNN=2.42–62.4 GeV. We use two different approaches to calculate the Λ polarization Py: approach I is based on equilibrium assumption so that Py is determined by thermal vorticity and approach II assumes a proportional relation between Py and the system's angular momentum in Λ's rest frame. We find that both approaches can describe the experimental data at low energies around a few GeV but only approach I describes well also higher-energy data. This suggests that at higher energies the relativistic effect plays an important role. After taking into such effect properly, the relativity-improved approach II can describe the higher-energy data as well.