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Mechanical exfoliation of plate-stratiform structured Bi12O17Cl2 by high-energy ball milling method is firstly presented to enhance photocatalytic performance. As-exfoliated Bi12O17Cl2 photocatalyst with milling time of 3 d is characterized as ultrathin nanosheets with thickness only in several nanometers. TEM observation proves gradual exfoliation process with grinding time increased. Photoluminescence spectra prove that emission intensity of as-exfoliated Bi12O17Cl2 with milling time of 3 d is suppressed, which implies that fast transfer and slower recombination of photogenerated electron-hole pairs are obtained. Adsorption ability and photodegradation efficiency of as-exfoliated Bi12O17Cl2 are greatly enhanced in comparison with original Bi12O17Cl2. For as-exfoliated Bi12O17Cl2 with milling time of 3d, the photocatalytic efficiency is more than 3 times as for the original Bi12O17Cl2. Based on the analysis of the photocatalytic property, the enhancement mechanism for as-exfoliated Bi12O17Cl2 photocatalyst is discussed considering the variation of the microstructure.