A novel 3D PANI/MgIn2S4 nanoflower photocatalyst was synthesized for the wastewater treatment and energy conversion. In all composites, 1% 3D PANI/MgIn2S4 nanoflower composite exhibits the top-flight photocatalytic activity, which can be nearly 100% reduction of Cr (VI) after 30 min and nearly 97.0% of methyl orange (MO) is degraded in 50 min. At the same time, it was proved that the Cr (VI) was completely reduced by liquid ultraviolet and XPS. The as-prepared 3D PANI/MgIn2S4 nanoflower composites showed a significantly increased photocatalytic performance for hydrogen production under visible light irradiation. The photocatalytic mechanism illustrates that the introduction of polyaniline can prominently enhance the light absorption and promote effective separation of electron-hole pairs of the composites. PL spectrum and Transient photocurrent response show excellent photoinduced charge separation efficiency possess in 3D PANI/MgIn2S4 nanoflower. Electron spin response (ESR) technique and active species trapping experiments confirmed that e(-), O-2 and % center dot O-2 -played a very important role in the process of photoreduction of Cr (VI) and h(+) and % center dot O-2(-) were the principal active groups participated in photo-degradation of methyl orange (MO). In addition, the degradation process is accompanied by the generation of singlet oxygen (O-1(2)) and hydrogen peroxide (H2O2), suggesting that these reactive oxygen species play a cardinal part in the photodegradation mechanism. The cycling and stable performance of 3D PANI/MgIn2S4 nanoflower also shows excellent photocatalytic effect. Such 3D PANI/MgIn2S4 nanoflower composite present a cracking strategy to enhance the photoactivity of the catalysts for wastewater treatment.