An effective strategy to enhance the performance of perovskite oxides is to precise adjust their sizes and forbidden bandwidth. However, it is challenging to synthesize perovskites with the desired morphology and structure because perovskites typically require harsh synthesis conditions, such as high pressure and high temperature. Herein, we designed and synthesized BiFeO₃/Bi₂₅FeO₄₀ heterojunction nanoparticles with adjustable energy-band structure using the simple sol-gel method by adjusting the concentration of Fe³⁺ and calcination temperature. As a result, the BiFeO₃/Bi₂₅FeO₄₀ heterojunction exhibits a lower band gap than BiFeO₃, Bi₂Fe₄O₉, Bi₂₅FeO₄₀, and BiFeO₃/Bi₂Fe₄O₉. The reduced band gap of BiFeO₃/Bi₂₅FeO₄₀ is due to the formation of heterojunction structure. Meanwhile, the three strategies of heterojunction construction, temperature controlling, and Fe³⁺ doping can precise regulate the band gap structure of bismuth ferrite (BFO). These findings not only provide guidance for the construction of perovskite heterostructures but also promote the application of BFO in photocatalytic and other redox reactions.