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Inorganic halide perovskite CsPbI₃ is highly promising in the photocatalytic field for its strong absorption of UV and visible light. Among the crystal phases of CsPbI₃, the δ-phase as the most aqueous stability; however, directly using it in water is still not applicable, thus limiting its dye photodegradation applications in aqueous solutions. Via adopting nitrogen-doped graphene quantum dots (NGQDs) as surfactants to prepare δ-phase CsPbI₃ nanocrystals, we obtained a water-stable material, NGQDs-CsPbI₃. Such a material can be well dispersed in water for a month without obvious deterioration. High-resolution transmission electron microscopy and X-ray diffractometer characterizations showed that NGQDs-CsPbI₃ is also a δ-phase CsPbI₃ after NGQD coating. The ultraviolet-visible absorption spectra indicated that compared to δ-CsPbI₃, NGQDs-CsPbI₃ has an obvious absorption enhancement of visible light, especially near the wavelength around 521 nm. The good dispersity and improved visible-light absorption of NGQDs-CsPbI₃ benefit their aqueous photocatalytic applications. NGQDs-CsPbI₃ alone can photodegrade 67% rhodamine B (RhB) in water, while after compositing with TiO₂, NGQDs-CsPbI₃/TiO₂ exhibits excellent visible-light photocatalytic ability, namely, it photodegraded 96% RhB in 4 h. The strong absorption of NGQDs-CsPbI₃ in the visible region and effective transfer of photogenerated carriers from NGQDs-CsPbI₃ to TiO₂ play the key roles in dye photodegradation. We highlight NGQDs-CsPbI₃ as a water-stable halide perovskite material and effective photocatalytic adjuvant.