Amorphous silicon oxide (a-SiOx) films are prepared using plasma enhanced chemical vapor deposition technique. The microstructural and optical characteristics of a-SiOx films are investigated by Fourier transformed infrared spectra, optical absorption spectra, and steady/time-resolved photoluminescence (PL) spectra. With the CO2 flow rate increasing, the band gap and PL intensity of the films increase, the PL peak moves toward high energies, and the full width at half maximum of the PL are widened. In addition, the decay time at PL peaks increases from 6.2 ns to 21 ns with oxygen content enhancing. However, linearly decreased decay time with energy increasing is found for one sample. Optical transitions among band tail states of amorphous materials are considered as the main light emission mechanism based on the analysis of optical absorption, emission and decay characteristics.