Heterojunction fabrication is a general and practical method for producing high-efficiency photocatalysts. The photocatalyst of S-type heterojunction greatly improves the carrier separation efficiency to a certain extent, thereby promoting the degradation efficiency. In this work, we reported a unique S-scheme heterojunction photocatalyst composed of n-type Cs3Bi2Br9 (CBB) and p-type BiOBr with well-defined junction interfaces for photocatalytic degradation of Rhodamine B (RhB). By soaking CBB in an aqueous alcohol solution, the desirable BiOBr/CBB heterostructure was created in-situ, forming the intimate interfaces and facilitating the separation of photogenerated charge carriers. Compared with the pristine CBB, a substantial enhancement in photocatalytic performance of BiOBr/CBB heterostructure was achieved through an optimized water treatment. The results showed that the degradation efficiency of the optimal BiOBr/CBB heterostructure reached 98.55 % in 20 min.