Room-temperature-fabricated amorphous Ga₂O₃ is an inexpensive and highly sensitive material for high-performance solar-blind ultraviolet (UV) (220–280 nm) detectors, which are extremely useful given the widespread use of solar-blind UV photoelectronic technology in multiple areas. The UV detection characteristics of room-temperature deposited amorphous Ga₂O₃ thin films fabricated by a simple RF magnetron sputtering method were studied, and the deposition Ar pressure of an a-Ga₂O₃ thin film was varied to determine the mechanism underlying high-performance UV detection and to develop an ideal a-Ga₂O₃ thin film for UV detection. A high-response amorphous Ga₂O₃-based UV detector was made at 0.5 Pa, and the maximum response of the device reached 436.3 A W⁻¹ under 240 nm UV light with a 25 V bias voltage, which is near the maximum values for a single-crystal β-Ga₂O₃ material deposited at a high temperature. An amorphous Ga₂O₃-based UV detector with a low I_dark noise level (4.9 nA at 25 V) and a high I_uv/I_dark ratio (107314.4) was made at 1.2 Pa, and the I_uv/I_dark ratio of the device was near that of a UV detector based on single-crystal β-Ga₂O₃ with a complex metal-oxide–semiconductor field-effect transistor (MOSFET) structure. Through a comparative analysis of the electrical characteristics and the gain mechanism within amorphous Ga₂O₃ thin films deposited at different Ar pressures, the high UV response of the amorphous Ga₂O₃ detector at 0.5 Pa is found to mainly result from the quasi-Zener tunneling multiplication phenomenon between different resistance components. The low I_dark value and the high signal-to-noise ratio of the amorphous Ga₂O₃-based detector deposited at 1.2 Pa were mainly due to more high-resistance components and a relatively higher tunneling gain in the device. In addition, the amorphous Ga₂O₃-based detectors showed a much shorter response time (0.08 μs for the device deposited at 0.65 Pa) and recovery time (t_d1 = 0.21 ms, t_d2 = 5.88 ms for the device deposited at 1.2 Pa) than the reported crystal semiconductor-based devices and multiple complex structure devices. Thus, the room-temperature-fabricated amorphous Ga₂O₃ thin film could quickly and effectively detect a faint UV light signal in the presence of a very noisy background, which is extremely important in the applications of UV detectors in wearable, flexible photoelectronic devices.