The ultrathin two-dimensional materials with the unique catalytic performance attract extensive interests of researchers. In this paper, ultrathin Bi4O5Br2 nanosheet (Bi4O5Br2-UN) is synthesized through precursor method by controlling the mole ratio of Bi(NO3)(3)center dot 5H(2)O and cetyltrimethyl ammonium bromide. The ultrathin structure of Bi4O5Br2-UN was confirmed by X-ray diffraction, atomic force miscroscopy and high-resolution transmission electron microscopy. UV-vis light irradiation, Bi4O5Br2-UN display excellent selective photocatalytic activity for CO2 conversion, which also was proved by isotope experiment. After 2 h reaction, the CO generation over Bi4O5Br2-UN reached 63.13 mu mol g(-1), which is about 2.3 times higher than that of bulk Bi4O5Br2 (27.56 mu mol g(-1)). Additionally, the CO selectivity of Bi4O5Br2-UN is more than 99.5% in the whole catalytic process. The enhanced CO2 adsorption, suitable band position and improved charge separation induce the outstanding photocatalytic performance of Bi4O5Br2-UN. The photocatalytic mechanism of CO2 conversion with Bi4O5Br2-UN was analyzed by the in situ Fourier transform infrared spectrometry (FT-IR).