Furthermore, we found that the resonance structure in the rate profile disappears when the coupling of the cavity mode to the cavity photon bath is turned off, in agreement with the discussion in Ref. 38. This indicates that the cavity bath also plays an indispensable role in cavity-induced rate modification. More details of the reaction dynamics in a lossless cavity are provided in the SI. In addition, all the above re- sults remain unchanged when the molecular dipole moment is pointed in the opposite direction of the light polarization, i.e. ⃗ µ(x) ⋅ ⃗ e = -x, as is clear from the symmetry of the Hamiltonian. The ratio k/k 0 (k is the rate in the resonant condition and k 0 is the reaction rate outside the cavity) is almost quadratic to the light-matter interaction strength η c , as found in Ref. 39. This implies that the cavity-induced rate enhancement is caused by a second-order process with respect to the light-matter interaction, i.e. (k -k 0 )/k 0 ∝ (η c ⃗ µ(x) ⋅ ⃗ e) . Bringing together all the pieces of the puzzle mentioned above, we propose a reaction mechanism that can well explain the appearance of the resonance, that is, the rate increases most pronouncedly when the cavity frequency is in the vicinity of the molecular vibrational frequency. This increase is attributed to a cavity-induced cotunneling process, as illustrated in Fig. (a), which is analogous to but more complicated than the solvent-induced cotunneling process.