<p>The development of Cs₂AgBiCl₆ perovskites has attracted considerable interest in photovoltaics and optoelectronics due to their low toxicity and good stability. However, the indirect nature of its bandgap leads to a low absorption coefficient, which remains a major limitation for solar cell applications. In this study, a series of 26 Cs₂AgInₓBi₁₋ₓCl₆ (x = 0, 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.875, and 1) models with tunable bandgaps are designed through In³⁺ doping of pristine Cs₂AgBiCl₆. The structural, electronic, and optical properties of the stable Cs₂AgInₓBi₁₋ₓCl₆ are systematically investigated using first-principles calculations to evaluate potential for photovoltaic and optoelectronic applications. The results demonstrate that the bandgap transitions successfully from indirect to direct with increasing In³⁺ concentration, yielding a series of perovskites with gradually varying bandgap. Among these, Cs₂AgInₓBi₁₋ₓCl₆ (x = 0.75) exhibits outstanding optical properties, including a high absorption coefficient, high dielectric constant, and low reflectivity. It is inferred that the distinctive band structure, characterized by a dispersive conduction band (CB), contributes significantly to the enhanced optical performance. This work provides deeper insight into the intrinsic electronic properties of In³⁺-doped Cs₂AgBiCl₆ perovskites and establishes a foundation for improving the optical characteristics of In/Bi-based double perovskites. Furthermore, it offers systematic theoretical validation for previously reported experimental results.</p>