Gallium oxide (Ga ₂O ₃) as a wide bandgap semiconductor material has attracted much attention in the field of solar blind ultraviolet photodetection recently. Localized surface plasmon resonance is considered an effective way to improve the performance of optoelectronic devices by decorating the materials with metal particles. Al nanoparticle is a potential candidate for ultraviolet photodetectors because its suitable resonance wavelength is located in the ultraviolet wavelength range. In this work, the preparation temperature of the Ga ₂O ₃thin film is reduced by ultraviolet photo-annealing, and the amorphous Ga ₂O ₃thin film is prepared by using the solution method. In addition, aluminum (Al) nanoparticles are used to decorate the surface of Ga ₂O ₃thin film through thermal evaporation of aluminum film and then annealing, which improves the optoelectrical performance in the solar-blind wavelength spectrum. The X-ray diffraction (XRD) is employed to demonstrate the amorphous Ga ₂O ₃thin film. The surface roughness and the diameters of Al nanoparticles are analyzed by atomic force microscope (AFM). When the thickness of the deposited Al layer is about 3–5 nm, Al particles with the diameters in a range of 2–3 nm can be obtained. When the thickness of the deposited Al layer is more than 7 nm, Al nanoparticles turn into the particles with non-identical diameters and distribute in the form of agglomeration. Transmittance spectrum and absorption spectrum are employed to characterize the optical properties. The bandgaps of different samples are obtained, and the results are consistent with the theoretical ones. The Ga ₂O ₃thin film shows excellent optoelectrical performance and solar blind rejection ratio. Under 254 nm illumination, the maximum photo-to-dark current ratio is 2.55×10 ⁴, and the rejection ratio of I _{254 nm}/ I _{365 nm}is 2.2×10 ⁴. The best responsivity and detectivity of prepared samples are 0.771 A/W and 1.13×10 ¹¹Jones, which are about 34 times and 36 times higher than those without Al decoration. The improvement is attributed to the scattering effect of surface Al nanoparticles for illumination, which enhances the Ga ₂O ₃thin film absorption of solar blind ultraviolet light. Meanwhile, owing to the localized surface plasma resonance, the local electromagnetic fields generated near the Al nanoparticles promote the transfer of carriers in the film. Furthermore, the point about how the Al nanoparticles decorated on the surface of Ga ₂O ₃thin film influence the time response is also discussed in this work. As a result, the introduction of defect states will lead to an increase of fall time due to the Al deposition. In this work, the effect of Al nanoparticles decoration on the optoelectrical performance of amorphous Ga ₂O ₃thin film solar blind ultraviolet photodetectors is studied based on a simple preparation, which provides a possible pathway for developing the flexible solar blind electronic devices in the future.