Halide perovskite-based photodetectors, because of their fundamental scientific importance and practical applications in the military and civil fields, have drawn worldwide attention in recent years. However, the toxicity and instability issues are major challenges for their mass production and commercialization. In this study, for the first time, we report the use of the one-step spin-coating method for the preparation of lead-free double perovskite Cs₂AgBiBr₆ thin films for photodetector applications. The morphology, crystallinity, and optical properties of the as-grown Cs₂AgBiBr₆ thin films were first investigated. Further, symmetrically structured photoconductive detectors were fabricated and characterized. The device performance was remarkable in terms of a high responsivity of 7.01 A W⁻¹, an on/off photocurrent ratio of 2.16 × 10⁴, a specific detectivity of 5.66 × 10¹¹ Jones, an external quantum efficiency of 2146%, and a fast response speed of 956/995 μs. More importantly, the unencapsulated photodetectors demonstrate remarkable operational stability over the aging test (36 h, 35–45% humidity), and the photodetection ability can be almost maintained. Moreover, after storage for two weeks in ambient air, the proposed photodetectors can be efficiently sustained, demonstrating remarkable stability against water and oxygen degradation. Our results indicate that lead-free double perovskite Cs₂AgBiBr₆ is potentially an environmentally friendly alternative to fabricate high-efficiency and stable perovskite photodetectors for practical applications.