The conversion efficiency and stability of photocatalysts play important roles in the full photocatalytic CO₂ reduction cycle. In this work, a simple hydrothermal and thermal treatment method was used to convert thin 3D flower-like CoAl-layered double hydroxides (CoAl-LDHs) into ultrathin flower-like 3D porous spinel CoAl₂O₄ (CoAl). The yields of the optimized CoAl-750 catalyst for CH₄ and CO after 7 h of illumination were 58.98 μmol g⁻¹ and 33.11 μmol g⁻¹, respectively. The CoAl-750 catalyst demonstrates high stability after 35 h of cycling, with the yields of CH₄ and CO being 107.19% and 98.38% of those in the first cycle, respectively. The CoAl spinel catalysts exhibit outstanding stability, while increasing the conversion efficiency of photocatalytic CO₂ reduction. The excellent catalytic performance is attributed to the ultra-thin porous 2D nanosheets and spinel structure of CoAl. This work is expected to open new avenues for the application of spinel CoAl₂O₄ in photocatalysis.