Triclosan (TCS) is considered a potential pollutant and it threatens human health and survival of other organisms. Bi7O9I3 was successfully synthesized with a simple solvothermal method. In this study, a novel n-n heterojunction photocatalyst (Bi7O9I3 /Bi5O7I) was formed by in situ calcination of Bi5O7I3. It was obvious that the morphology had changed in the calcination process from the original sheet structure to the bone-stick-like structure, but still maintained a spherical 3-dimensional structure. At the same time, two kinds of lattice fringes were identified from the HR-TEM in sample Bi7O9I3-450 which effectively proved the formation of the Bi7O9I3/Bi5O7I heterojunction. This composite photocatalyst could effectively remove TCS from the water. Its photocatalytic efficiency was significantly improved compared with the pure Bi7O9I3. This was attributed to the internal electric field in the composite catalyst which facilitated the transport and migration of photo-generated carriers. The degradation mechanism of TCS was analyzed, and the electron reduction and center dot O-2(-) species and singlet oxygen oxidation played essential role in the TCS degradation by radical scavenger experiment. These species could react with TCS pollutants, leading to the degradation of TCS. Finally, the eleven degradation products were identified by the HPLC-MS method, and the two major reaction pathways were deduced.