Low-dimensional lead halides have attracted increasing attention due to their potential application as single-component white-light emitters. These materials exhibit a complex emission spectral structure, ranging from free exciton narrowband emissions to self-trapped exciton broadband emissions. However, there is still no consensus for the underlying physical mechanism, especially in the spectrum with both narrowband and broadband emissions. Here we aim to elucidate the correlation between the emission spectrum and the exciton-phonon coupling in the mixed halide perovskite BAPb(BrCl). Our findings reveal that the interplay between exciton localization and delocalization results in an intermediate exciton-phonon coupling, leading to line shapes beyond the Huang-Rhys model for the self-trapped exciton. By incorporating the exciton motional effect, we establish a unified photophysical model describing the emission spectrum from the self-trapped exciton type to the free exciton type. These results provide essential insights into the mechanisms governing exciton-phonon interactions and offer ways to control white-light emission in two-dimensional perovskites.