Constructing a heterojunction and designing a band gap are effective methods to promote the efficiency of a photocatalyst and solar energy utilization. In this work, g-C3N4/Yb3+-Bi5O7I nanostructured photocatalysts were successfully prepared using a hydrothermal method with glycol as a solvent and a calcination method, and the performance of the photocatalysts containing different amounts of g-C3N4 was studied. A series of characterizations confirmed the successful complexation of g-C3N4 nanosheets with Bi5O7I, and Yb3+ was successfully doped into the lattice of Bi5O7I. g-C3N4 and Yb3+ have a synergistic effect on expanding the photoresponse range of CYB(g-C3N4/Yb3+-Bi5O7I) composite photocatalysts. As expected, CYB samples have wonderful photocatalytic performance and stability in the removal of Hg-0, among which CYB-20 has the highest removal efficiency, reaching 79%. Yb3+ with upconversion characteristics can convert infrared light into visible light and form new energy levels in g-C3N4 and Bi5O7I form a Z-scheme heterojunction to improve the photocatalytic performance of Bi5O7I. Besides, the electron and hole transmission paths and the catalytic mechanism of the photocatalysts are also proposed. This work offers a new approach to construct a photocatalyst system for pollutant emission control with a wide wavelength range of sunlight, which may be helpful to develop environmentally benign functional materials.