Recent experiments have shown that chemical reaction rates can be suppressed or enhanced by resonantly coupling molecular vibrations to quantized radiation modes inside a Fabry–Pérot (FP) microcavity. This effect has the potential to selectively slow down competing reactions or speed up a target reaction, offering a paradigm shift in chemistry. However, the fundamental mechanism and theoretical understanding of cavity-modified ground-state chemical kinetics remain elusive. This work aims to develop a microscopic theory to explain these observed vibrational strong coupling (VSC) effects, particularly focusing on the resonance effect under normal incidence. The resonance effect occurs when the cavity frequency matches the bond vibrational frequency, $\omega_c = \omega_0$, and only happens at the normal incidence ($k_{\parallel} = 0$). The collective effect, where the magnitude of VSC modification increases with the number of molecules, and the driving by thermal fluctuations without optical pumping are also observed. The isotropic disorder of dipoles in the cavity is assumed in experiments with many molecules. The authors generalized their analytic Fermi’s golden rule (FGR) rate theory of VSC to incorporate many molecules and cavity modes for both 1D and 2D FP cavities. They evaluated the photonic mode density of states (DOS) inside a 1D FP cavity and found a van-Hove-type singularity at $k_{\|} = 0$. For a 2D FP cavity, the modified photon mode DOS remains dominant around the bottom of the dispersion band where $k_{\|} = 0$, which is crucial for explaining the normal incidence condition of the VSC-modified chemical reaction rate constant. The theoretical results provide a possible explanation for the resonance condition and why only at the normal incidence angle there is a resonance effect. The resonance behavior of $k_{\text{VSC}}$ is also discussed, showing that the VSC-modified rate constant occurs only when $\omega_c = \omega_0$. This is due to the van-Hove singularity in the 1D DOS, which forces the integral to survive only at $\omega = \omega_c$. For a 2D cavity, the VSC-modified rate constant is still maximized around $\omega_c = \omega_0$, fulfilling the normal incidence condition. The collective effect is not explicitly present in the current theory, which limits its applicability to small $N$ and strong coupling between molecules and the cavity mode. Future work is needed to consider multiple excitations and the rate constant theory in this scenario.Recent experiments have shown that chemical reaction rates can be suppressed or enhanced by resonantly coupling molecular vibrations to quantized radiation modes inside a Fabry–Pérot (FP) microcavity. This effect has the potential to selectively slow down competing reactions or speed up a target reaction, offering a paradigm shift in chemistry. However, the fundamental mechanism and theoretical understanding of cavity-modified ground-state chemical kinetics remain elusive. This work aims to develop a microscopic theory to explain these observed vibrational strong coupling (VSC) effects, particularly focusing on the resonance effect under normal incidence. The resonance effect occurs when the cavity frequency matches the bond vibrational frequency, $\omega_c = \omega_0$, and only happens at the normal incidence ($k_{\parallel} = 0$). The collective effect, where the magnitude of VSC modification increases with the number of molecules, and the driving by thermal fluctuations without optical pumping are also observed. The isotropic disorder of dipoles in the cavity is assumed in experiments with many molecules. The authors generalized their analytic Fermi’s golden rule (FGR) rate theory of VSC to incorporate many molecules and cavity modes for both 1D and 2D FP cavities. They evaluated the photonic mode density of states (DOS) inside a 1D FP cavity and found a van-Hove-type singularity at $k_{\|} = 0$. For a 2D FP cavity, the modified photon mode DOS remains dominant around the bottom of the dispersion band where $k_{\|} = 0$, which is crucial for explaining the normal incidence condition of the VSC-modified chemical reaction rate constant. The theoretical results provide a possible explanation for the resonance condition and why only at the normal incidence angle there is a resonance effect. The resonance behavior of $k_{\text{VSC}}$ is also discussed, showing that the VSC-modified rate constant occurs only when $\omega_c = \omega_0$. This is due to the van-Hove singularity in the 1D DOS, which forces the integral to survive only at $\omega = \omega_c$. For a 2D cavity, the VSC-modified rate constant is still maximized around $\omega_c = \omega_0$, fulfilling the normal incidence condition. The collective effect is not explicitly present in the current theory, which limits its applicability to small $N$ and strong coupling between molecules and the cavity mode. Future work is needed to consider multiple excitations and the rate constant theory in this scenario.