The stereoselective polymerization of polar vinyl monomers has recently received much attention due to their excellent physicochemical properties. Over the past decade, breakthroughs have been achieved in this field by rare-earth catalysts. However, the mechanistic origins of those stereoselective polymerizations still remain unclear. Herein, stereoselective polymerization of ortho-methoxystyrene (oMOS) by several representative rare-earth catalysts bearing different ligands (i.e., η5-C5Me5, pyridinyl-methylene-fluorenyl, quinolyl-anilido, β-diketiminato) were systematically investigated by density functional theory (DFT) calculations. After achieving agreement between the calculations and experiments, we focused on discussing the role of ligands in controlling stereoselectivity. Our results reveal that the stereoregularity of oMOS polymerization is mainly controlled by the steric effect of the catalyst-monomer structures. Specifically, the type of ligand influences the orientation and configuration of the inserting monomer, thereby affecting the tacticity of the polymers. In the cases of η5-C5Me5-, pyridinyl-methylene-fluorenyl, and quinolyl-anilido-ligated yttrium catalysts, we observe consistent insertion directions and alternating insertion sides of oMOS monomers, leading to syndiotactic selectivity. The opposite insertion directions and the alternating insertion sides of oMOS monomers were observed in the case of the β-diketiminato yttrium catalyst, leading to isotactic selectivity. These findings reported here offer valuable insights into the role of ligands in controlling stereoselectivity in rare-earth catalyzed coordination polymerization of polar vinyl monomers, thus providing guidance for the rational design of new ligands for stereospecific polymerization of polar monomers in the future.