Designing photocatalysts with high light utilization and efficient photogenerated carrier separation for pollutant degradation is one of the important topics for sustainable development. In this study, hierarchical core–shell material α-Fe₂O₃@ZnIn₂S₄ with a step-scheme (S-scheme) heterojunction is synthesized by in situ growth technique, and MXene Ti₃C₂ quantum dots (QDs) are introduced to construct a double-heterojunction tandem mechanism. The photodegradation efficiency of α-Fe₂O₃@ZnIn₂S₄/Ti₃C₂ QDs to bisphenol A is 96.1% and its reaction rate constant attained 0.02595 min⁻¹, which is 12.3 times that of pure α-Fe₂O₃. Meanwhile, a series of characterizations analyze the reasons for the enhanced photocatalytic activity, and the charge transport path of the S-scheme heterojunction/Schottky junction tandem is investigated. The construction of the S-scheme heterojunction enables the photo-generated electrons of α-Fe₂O₃ and the holes of ZnIn₂S₄ to transfer and combine under the action of the reverse built-in electric field. Due to the metallic conductivity of Ti₃C₂ QDs, the photogenerated electrons of ZnIn₂S₄ are further transferred to Ti₃C₂ QDs to form a Schottky junction, which in turn forms a double-heterojunction tandem mechanism, showing a remarkable charge separation efficiency. This work provides a new opinion for the construction of tandem double heterojunctions to degrade harmful pollutants.