Layered bismuth oxyhalides with bilayered (Bi₂O₂) fluorite (FL) slabs are promising visible-light photocatalysts because of their excellent stability and the ability to adjust band levels depending on the layers combined. It is interesting to manipulate the Bi₂O₂ slab itself, but only trilayered FL blocks (e.g., Bi₃O₄) are reported so far. Here, a structurally uncharacterized Bi₁₂O₁₇Cl₂ , which is extensively studied as a photocatalyst for a variety of reactions, has a sextuple Bi₆O _8.5 block separated by Cl is shown. Unlike double and triple layered cases, the inner region of the Bi₆O_8.5 block contains 1D rock-salt (RS) units in the FL matrix along the a-axis, causing in-plane corrugation. A topo-chemical reaction involving anion-exchange gives Bi₁₂O_17-0.5xF_xCl₂ (x ≤ 6) with alternate FL and RS slabs along the c-axis. The elimination of the structural corrugation increases higher photo-conductivity and improves photocata-lytic activity against acetic acid decomposition under visible light irradiation. This study paves new opportunities of controlling the properties of layered bismuth oxyhalides by the thickness of Bi-O block, FL/RS configuration, and structural modulation.