In this method, CuGaO2 thin films were prepared by a sputtering method and annealing treatment under a controlled nitrogen atmosphere to investigate the relationship between the structural changes and optoelectronic properties of Cu–Ga–O thin films. The amorphous Cu–Ga–O thin films exhibited a low surface roughness, high resistance, and a high photon absorption capacity. The thin film with composite phases of CuO, CuGa2O4, and CuGaO2 structures did not exhibit dominant absorption characteristics at a specific wavelength; the bandgap of the material was 2.44 eV, making it a high-resistance thin film. The formation temperature of single-phase CuGaO2 was 750 °C. CuGa2O4 and CuO reacted to form CuGaO2. CuGaO2 exhibited a high-temperature phase, and a polygon-like microstructure was observed in the thin-film cross section. With increasing annealing temperature, the surface roughness, average grain size, direct bandgap, and resistivity of the CuGaO2 thin films increased accordingly. The surface roughness of the CuGaO2 thin films ranged from 5.7 nm to 7.3 nm, with average grain sizes ranging from 35.5 nm to 39.0 nm. The direct band gaps of CuGaO2 annealed at 750 °C, 800 °C, 850 °C, and 900 °C were 3.45 eV, 3.50 eV, 3.64 eV, and 3.65 eV, respectively; their corresponding resistivities were 57.4 Ω cm, 78.9 Ω cm, and 121.7 Ω cm, respectively. At a higher annealing temperature (900 °C), the thin film was unable to retain single- phase CuGaO2, instead forming a secondary phase of GaO.