Zero-dimensional (0D) lead-free halide perovskites have lately received significant interest owing to their captivating broadband emissions. An in-depth understanding of the luminescence mechanism of self-trapped excitons (STEs) and realization of effective regulation of luminescence properties have become a major challenge in the research of lead-free metal halides. Herein, we have synthesized the Cs2ZnCl4 and Sb3+-doped Cs2ZnCl4 crystals and conducted a comprehensive investigation into their distinct electronic structures and optical characteristics. The findings from both experimental and theoretical investigations indicate that the tricolor luminescence in Cs2ZnCl4 and blue emission in Sb3+-doped Cs2ZnCl4 stem from intrinsic STEs, and the near-infrared emission originates from extrinsic STEs associated with the Sb3+ ion in Sb3+-doped Cs2ZnCl4. Sb3+ doping increases the quantum yield of Cs2ZnCl4 to a large extent. In addition, intersystem crossing, exciton self-trapping, and lattice relaxation are the main reasons for the large Stokes shift. The present study is expected to provide a novel perspective for researchers in comprehending the luminescent mechanism of STEs and advancing the utilization of 0D lead-free metal halides in optoelectronic applications.