The mechanism of the Ru(salen)-mediated (salen = 1,2-cyclohexanediamino-N,N'-bis(3,5-di-tert-butylsalicylidene)) cyclopropanation reaction of styrene with ethyl diazoacetate (EDA) is explored with density functional theory (DFT) methods. Five proposed reaction pathways, including (1) a stepwise process containing the formation of carbene species and cyclopropanation step (path a), (2) a one-step process (path b), (3) a bis-carbene mechanism (path c), (4) a three-centered-intermediate pathway (path d), and (5) the main side reaction of dimerization of EDA (path e), are taken into consideration to determine the most favorable mechanism. Computational results indicate that path a with a barrier of 27.9 kcal/mol (trans) is superior to all other pathways. The geometries of the critical transition states are picked out for further analyses. It is found that the C-H-a group of the catalyst plays a key role in enantioselectivity. The destruction of the active center (for example, a methyl group substituent) can dramatically decrease the catalytic efficiency. In addition, the axial ligands are found to mediate energy barriers of the formation of carbene species and the cyclopropanation step in entirely opposite directions. The natural bond orbital (NBO) analyses demonstrate that carbene species show different characteristics of Fischer- or/and Schrock-type complexes. This study may help to design and develop more efficient catalysts for metal-mediated cyclopropanation reactions.