Exploring and utilizing novel materials with self-trapped excitons (STEs) is highly desired due to their unique physical properties and multiple optoelectronic applications. Here, a 2D SnPS crystal is reported with obvious STEs emission caused by distorted [SnS] and [PS] octahedral units and strong electron-phonon coupling. The STEs feature wide photoluminescence (PL) spectra range from 600 to 850 nm and a large Stokes redshift of ≈0.6 eV. Carrier dynamics measurements including temperature-dependent PL and transient absorption reveal a large Huang–Rhys factor of ≈18.3 and a self-trapping process of ≈10–10 ps in the 2D SnPS crystal. Such self-trapped states enable 2D SnPS crystal a wide spectral response range and excellent photodetection performance. As a result, high responsivity (22.8 A W) and detectivity (3.98 × 10 Jones) are achieved under 365 nm light illumination. These results provide a deep insight into the photophysical process of STEs, which lays the foundation for developing novel STEs-based materials and optoelectronic devices.