The temperature-dependent conductivity of Mg-doped amorphous silicon film on sapphire substrate was studied by terahertz (THz) time-domain spectroscopy. The complex conductivity showed characteristics of charge localizations. The complex conductivity was fitted by the Drude-Smith model. The relaxation time increased at higher temperatures implying that mobility also increased. Lastly, plasma frequency, proportional to carrier concentration, appeared to be constant with temperature.