As a direct Z-scheme photocatalyst, a two-dimensional/two-dimensional (2D/2D) Bi2O2CO3/Bi4O5Br2 (BOC/BOB) heterostructure was fabricated by stacking the ultrathin nanosheets of Bi2O2CO3 and Bi4O5Br2 via a simple one-step hydrothermal synthesis process. The atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM) results show that the heterostructure was successfully formed by coupling the ultrathin nanosheets of BOC and BOB. By optimizing the content of BOC, we found that the 30% BOC/BOB composite can exhibit a superior photocatalytic activity (53.2%) for NOx removal under simulated solar light illumination, which is much higher than that of single-phase BOC (20.4%) or BOB (37.9%). The results from trapping experiments and DMPO-ESR spin-trapping measurements demonstrate that both center dot O-2(-) and center dot OH are the main active species during the photocatalytic reaction process. According to the DFT calculations, a Z-scheme heterojunction is formed between the BOC and BOB ultrathin nanosheets, in which the photo-induced electrons in BOC are combined with holes in BOB. This process can effectively hinder the recombination of photo-induced charge carriers in the BOC/BOB nanocomposites and resulting in the enhancement of charge separation efficiency. This work may shed light on developing more efficient photocatalysts by designing the 2D/2D Z-scheme heterostructures using ultrathin nanosheets.