The light-illumination stability of amorphous InGaZnO Thin Film Transistors (a-IGZO TFTs) in oxygen and moisture ambience was in-depth characterized by both current-voltage (I-V) and capacitance-voltage (C-V) measurements. With the illuminated light wavelength decreasing, both I-V and C-V curves shifted negatively. When the ambient oxygen content or moisture level increased, the a-IGZO TFTs exhibited more stable properties under light illumination. A qualitative model was proposed to explain the related physical mechanism. The higher oxygen content or moisture level benefited the light-illumination-induced oxygen adsorption at back channels of a-IGZO TFTs and prevented the formation of oxygen vacancies (V-o) in channel layers; the V-o variation with the light illumination became more difficult and hence led to better light-illumination stability of the corresponding TFT devices.