In2O3-CuO composites were prepared by hydrothermal method using InCl3.4H2O and Cu(NO3)2.3H2O as raw materials and urea as precipitant. The gas-sensitive properties and sensing mechanism of UV-activated In2O3-CuO composites were investigated by correlating microstructural characterization and macroscopic gas-sensitive properties. The results show that the sensitivity of the In2O3-CuO composite is 298 for 50 ppm formaldehyde gas at room temperature (25 °C) under 375 nm UV light irradiation, which is 124 times more sensitive compared with pure In2O3 (2.4), and the great improvement of the gas-sensitive performance is attributed to the p-n heterojunction formed by In2O3 and CuO, and the synergistic photoactivation conditions produce a heterojunction interface of photogenerated electrons-hole with oxygen species (O2 and O2-) establishes a photoactivated adsorption-desorption cycle of oxygen, resulting in enhanced gas adsorption-desorption process and surface reaction of the material at room temperature. It provides a new strategy for the application of photosensitive materials and the design of room-temperature gas sensing materials.