Ultra-wide bandgap semiconductor gallium oxide (Ga₂O₃) has important applications in power electronics and information sensing, its efficient and economical preparation is important to realize its industrial promotion. In this paper, a Sn-assisted mist chemical vapor deposition (mist CVD) technique is reported, based on which high-quality pure-phase α-Ga₂O₃ thin films were successfully epitaxially grown on c-plane sapphire substrates by this non-vacuum, low-cost method. The mist CVD growth temperature regulation experiment shows that the temperature for epitaxial growth of pure-phase α-Ga₂O₃ thin films is between 500 and 600 ℃. The physical phase, morphology, optical features, elemental content and valence of the pure-phase α-Ga₂O₃ thin films were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-visible spectrophotometer, and X-ray photoelectron spectrometer (XPS) techniques. The results indicate that the α-Ga₂O₃ thin films grown at the temperature of 600 ℃ possess a higher degree of crystallinity, a denser and flatter surface morphology. Further, the deep-UV (DUV) photoresponse properties of α-Ga₂O₃ thin films were investigated by constructing photodetectors with metal-semiconductor-metal (MSM) structures. For α-Ga₂O₃ thin films prepared at 500 and 600 ℃, the photodetectors show photo-to-dark current ratios (PDCR) of 5.85×10⁵ and 7.48×10³, external quantum efficiencies (EQE) of 21.8% and 520%, and responsivities of 0.044 and 1.09 A/W, respectively. Under 6 V bias voltage and 254 nm illumination, the response time is 0.97/0.36 s for α-Ga₂O₃ thin film grown at 500 ℃, while it increases to 2.89/4.92 s for the sample grown at 600 ℃, which may be ascribed to the formation of donor impurities within the α-Ga₂O₃ thin films by Sn-assisted growth, and affecting the carrier transport efficiency.