The soft and polar nature of quasi‐2D (PEA)2PbBr4 perovskite, and robust photo‐generated excitons lead exciton‐polaritons and exciton‐polarons as the important phenomena near the band edge for application in the lighting aspect. In this work, a convenient methodology is proposed based on the polariton resonant modes in temperature‐dependent (77 K to RT) spectroscopy, and investigate the effect of these quasi‐particles on refractive index dispersion. The large binding energy (≈335 meV) of quasi‐2D excitons is obtained by the reflectance measurements at 77 K. Stable exciton‐polaritons and exciton‐polarons are confirmed by energy dispersions and the observation of self‐trapped exciton‐polaron state, respectively. Furthermore, the large negative thermal‐optic coefficient due to damping effect of exciton‐phonon scattering is observed. The phenomenon is opposite to those observed in conventional semiconductors (e.g., Si, Ge, GaN, AlN, GaAs, AlAs, and ZnO etc.). The observed stable negative thermal‐optic coefficients from 160 K to RT indicate that the quasi‐2D perovskite can be used as a phase compensator for conventional semiconductor materials. The shrinkage of the energy difference between lower polariton branches (LPs) and upper polariton branches (UPs) proves that oscillator strength decreases when the temperature rises from 77 to 300 K. Therefore, the strong damping effect of exciton‐phonon interactions reduces the oscillator strength when the temperature rises, and further result in the negative thermal‐optic behaviors of quasi‐2D (PEA)2PbBr4 perovskite.