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In this paper, a composite of tin diselenide (SnSe₂) functionalized by graphite-phase carbon nitride (g-C₃N₄) was successfully prepared by a hydrothermal method, and was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). These microstructure characterization results verified the successful synthesis of a multilayer g-C₃N₄/rod-shaped SnSe₂ composite. The gas sensitivity results showed that when the g-C₃N₄ ratio was 30%, the g-C₃N₄/SnSe₂ composite sensor had the highest response (28.9%) at 200 °C to 20 ppm sulfur dioxide (SO₂) gas, which was much higher than those of pristine g-C₃N₄ and SnSe₂ sensors at the optimum temperature. A series of comparative experiments proved that the g-C₃N₄/SnSe₂ composite sensor demonstrated an excellent response, strong reversibility and good selectivity for ppm-level SO₂ gas detection. The possible SO₂ sensing mechanism was ascribed to the heterostructure between the n-type SnSe₂ and n-type g-C₃N₄ nanomaterials. Furthermore, we also proposed the influence of the special structure of the g-C₃N₄ functionalized SnSe₂ composite on the gas-sensing characteristics.