The MoS2/Ag/AgBr composite with ultrathin-MoS2 layers was prepared by the methods of ultrasonic liquid exfoliation and ion exchange. The composition and micromorphology of ultrathin-MoS2/Ag/AgBr were detected by XRD, EDS, SEM and TEM in detail. The element mapping showed that the samples were well-distributed. The MoS2 in samples and the number of plies were detected by Raman spectra specifically. The results indicated that the photocatalytic performance of the as-prepared samples would increase by 50% when the average thickness of MoS2 was about 4 nm. The enhanced photocatalytic performance of ultrathin-MoS2/Ag/AgBr composite is attributed to the modified conduction band of MoS2 by reducing the layers of MoS2 to fit the potential of O-2/O2 center dot-. In particular, this work would expand the scope of application of MoS2 in photodegradation.