This paper presents a theoretical analysis of the resonance effect in vibrational strong coupling (VSC) chemistry at normal incidence. The theory explains how cavity modes enhance the transition from the ground state to the vibrational excited state of a reactant, which is the rate-limiting step in the reaction. Using Fermi's golden rule (FGR), the rate constant for many molecules coupled to many cavity modes inside a Fabry–Pérot (FP) microcavity is formulated. The theory provides a possible explanation for the resonance condition of the observed VSC effect and explains why only at normal incidence there is a resonance effect, whereas for oblique incidence, there is no apparent VSC effect. The current theory cannot explain the collective effect when a large number of molecules are coupled to the cavity, and future work is needed to build a complete microscopic theory to explain all observed phenomena in VSC. The paper discusses the resonance effect at normal incidence, which occurs when the cavity frequency matches the bond vibrational frequency, and only happens when the in-plane photon momentum is zero. The theory also explains the collective effect, which is the increase in the magnitude of VSC modification when increasing the number of molecules. The paper also discusses the driving by thermal fluctuations without optical pumping and the isotropic disorder of the dipoles in the cavity. The paper presents a microscopic theory to explain the observed VSC effects, especially focusing on understanding the resonance effect under normal incidence. The theory is based on the hypothesis that cavity modes enhance the transition from the ground state to the vibrational excited state of the reactant. The paper also discusses the formation of Rabi splitting and the role of the cavity lifetime in the VSC effect. The paper concludes that the VSC-modified rate constant is maximized at normal incidence, which is consistent with experimental observations. The theory provides a step forward towards understanding the fundamental difference between the condition for forming the Rabi splitting and that of the VSC resonance modification of the rate constant.This paper presents a theoretical analysis of the resonance effect in vibrational strong coupling (VSC) chemistry at normal incidence. The theory explains how cavity modes enhance the transition from the ground state to the vibrational excited state of a reactant, which is the rate-limiting step in the reaction. Using Fermi's golden rule (FGR), the rate constant for many molecules coupled to many cavity modes inside a Fabry–Pérot (FP) microcavity is formulated. The theory provides a possible explanation for the resonance condition of the observed VSC effect and explains why only at normal incidence there is a resonance effect, whereas for oblique incidence, there is no apparent VSC effect. The current theory cannot explain the collective effect when a large number of molecules are coupled to the cavity, and future work is needed to build a complete microscopic theory to explain all observed phenomena in VSC. The paper discusses the resonance effect at normal incidence, which occurs when the cavity frequency matches the bond vibrational frequency, and only happens when the in-plane photon momentum is zero. The theory also explains the collective effect, which is the increase in the magnitude of VSC modification when increasing the number of molecules. The paper also discusses the driving by thermal fluctuations without optical pumping and the isotropic disorder of the dipoles in the cavity. The paper presents a microscopic theory to explain the observed VSC effects, especially focusing on understanding the resonance effect under normal incidence. The theory is based on the hypothesis that cavity modes enhance the transition from the ground state to the vibrational excited state of the reactant. The paper also discusses the formation of Rabi splitting and the role of the cavity lifetime in the VSC effect. The paper concludes that the VSC-modified rate constant is maximized at normal incidence, which is consistent with experimental observations. The theory provides a step forward towards understanding the fundamental difference between the condition for forming the Rabi splitting and that of the VSC resonance modification of the rate constant.