Density functional theory (DFT) calculations were performed to study the detailed reaction mechanism of the cross-metathesis of ethylene and 2-butylene over heterogeneous Mo/HBeta catalysts. The whole process is divided into two stages: the initiation of Mo–carbene species from Mo-oxo precursors and the propagating process by these active sites to yield propylene. The formation of initial Mo–carbene takes place via the endothermic addition and the subsequent decomposition of the oxametallacyclobutane intermediate. In the propagating stage to yield the final products, MoCHCH3 firstly reacts with ethylene to form MoCH2, which would further react with 2-butylene to give another propylene molecule. The oxidation states of the Mo species have great influences on the reactivities associated with these two stages. It is unfavorable for MoIV-oxo precursors to produce Mo–alkylidene species compared with MoVI and MoV sites. The energy barriers indicate that the MoVI and MoV–alkylidene species could catalyze the olefin metathesis reaction, but MoV ones are more preferred to be the active sites. The calculation results are consistent with our previous XPS results.