Metal-free boron Lewis acids, tris(pentafluorophenyl)borane B(C6F5)3, have the advantages of low toxicity and low cost and are a promising catalyst. A density functional theory (DFT) calculation was used to clarify the mechanism and the origin of the diastereoselective cyclopropanation of aryldiazodiacetate and styrene derivatives catalyzed by B(C6F5)3. Four pathways were calculated: B(C6F5)3-catalyzed N-, C-, and O-bound boron-activated aryldiazodiacetate and without B(C6F5)3 catalysis. By calculating and comparing the energy barriers, the most possible reaction mechanism was proposed, that is, first, B(C6F5)3 catalyzed O-bound boron to activate aryldiazodiacetate, followed by the removal of a N2 molecule, and finally, styrene nucleophilic attack occurred to produce [2+1] cyclopropane products. N2 removal is the rate-limiting step, and this step determines the preference of a given mechanism. The calculated results are in agreement with experimental observations. The origin of diastereoselectivity is further explained on the basis of the favorable mechanism. The steric hindrance interference between the styrene aryl group and the large tri(pentafluorophenyl)borane B(C6F5)3 and the favorable π-π stacking interaction between the benzene rings combined to cause the high diastereoselectivity, which resulted in lower energy of the transition state (TS) corresponding to the reaction mechanism. The calculated results not only provide a more detailed explanation of the mechanism for the experimental study but also have certain reference and guiding significance for other catalytic cyclopropanation reactions.