In this work, we follow the inspiration from Ref. 25 and perform numerically exact simulations to investigate the VSC enhancement effect when coupling a single molecule inside a cavity. As opposed to the early work that treats the photonic DOF qc using Fock states and as part of the quantum subsystems, here, we use an effective spectral density theory to describe qc and cavity loss as an effective bath that couples to the reaction coordinate through the molecule-cavity interactions. Because of the exact nature of HEOM, including qc inside the bath should not influence the description of quantum dynamics. This effective spectral density treatment not only provides computational efficiency (because the quantum subsystem is only the molecular reaction coordinate or equivalently the quantum vibrational states) that allows for a much faster convergence for the calculation but also provides an intuitive understanding of the cavity mode as a "ratepromoting vibration (RPV) mode", which will enhance the rate constant if its frequency ω c matches the transition frequency ω 0 . We found that the key mechanism lies in the population being pumped to the vibrationally excited state in the reactant well, which quickly tunnels to the vibrationally excited state in the product well and then relaxes to generate the vibrational ground state in the product well.