Bi2MoO6 has emerged as a powerful visible-light-active oxidation photocatalyst with decent photocatalytic activity although further modifications are still essential to overcome its inherent high charge recombination efficiency. Among them, the strategy of constructing Bi2MoO6-based heterojunction has been demonstrated to be effective. In this work, a novel two-dimensional (2D) Bi2MoO6/Bi2S3 heterostructure was in situ fabricated via an anion exchange process, wherein [MoO6]2− layers were substituted by S2− ions. With simply tuning the concentration of S source (thiourea), the content of Bi2S3 can be easily controlled. The formed heterogeneous materials with appropriate Bi2S3 amount were found to substantially boost the yield of evolved oxygen under visible light irradiation, whereas excessive Bi2S3 is detrimental to the photoactivity. Comprehensive analysis disclosed a complementary band alignment between the two materials, whereby a step-scheme heterojunction was formed. The charge transfer mechanism was carefully proposed based on diverse characterization. Photogenerated electrons on Bi2MoO6 recombined with the holes on Bi2S3, while maintaining the strong oxidative capability of Bi2MoO6 and reductive capability of Bi2S3. The enhanced activity was attributed to the suppressed electron-hole recombination on the single catalyst, and the promoted charge transfer across the junction interface was demonstrated to be beneficial for better performance. Additionally, high concentration of Bi2S3 can inevitably cover the oxidation-active component, Bi2MoO6, which in turn decreased the photocatalytic performance.