As the first and essential step of the C-4 alkylation reaction, the formation of tertiary butyl carbocation over H-BEA zeolite was first systematically studied utilizing density functional theory (DFT) calculation. Herein, two reaction pathways (i.e., skeletal isomerization and hydride transfer) were systematically compared involving the Gibbs free energy at 350 K. It is found that the overall apparent reaction barrier of the hydride transfer pathway is 76 kJ/mol, much lower than that of skeletal isomerization pathway (92 kJ/mol), indicating that the hydride transfer pathway is predominant over the H-[Al]-BEA. Moreover, effect of acid strength on reaction mechanism was further investigated over H-[Al]-BEA and H-[B]-BEA catalysts with stronger and weaker acidity, respectively. Results show that the hydrogen transfer route is still the dominant pathway to generate tertiary butyl carbocation when acid strength changes, which correlates well with the experimental results. However, rate-determining steps of skeletal isomerization and hydride transfer are changed, and the higher Gibbs free energy barriers are obtained by decreasing the acid strength. The insights in this work will be of great significance to understand the initial C4 alkylation mechanism and to design highly efficient catalysts.