Organic–inorganic halide metal perovskites are an exciting class of two-dimensional (2D) materials that have sparked renewed interest for next-generation optoelectronics. In particular, the self-trapped excitons (STEs) in 2D perovskite with excellent optical properties suggest great potential in display and narrowband detection. A prerequisite of understanding STEs’ properties is correct identification of the underlying interaction that leads to STEs. Here, the optical properties of STEs in ${(\mathrm{iso}-\mathrm{BA})}_2{\mathrm{PbI}}_4$ are characterized through laser spectroscopy at various temperatures and excitation intensities. It is found that STEs are related to the octahedral distortion caused by strong electron–phonon interaction. Trapping and detrapping between STEs and free excitons (FEs) are clearly observed. With the increase in temperature, STEs and FEs will gain enough energy and migrate to each other. Moreover, by characterizing the thickness-dependent and two-photon excitation emission, it is confirmed that STEs exist inside the material because of their weak absorption. Our findings are of great significance for not only the fundamental understanding of STEs, but also the design and optimization of 2D-perovskite-based electronic and optoelectronic devices.