The structure of large aperture active mirror slab amplifier is presented, which is pumped by Xe-lamps. The application of active mirror in Nd: glass laser amplifier is explored. The size of Nd:glass is 890 mm×450 mm×50 mm . Doping density is 1.2% of mass fraction. The small signal gain coefficient and energy storage efficiency of different voltages are measured at the best delay time. The energy storage efficiency is 1.94% and the small signal gain coefficient is 0.0371 cm-1 when the charging voltage is 22 kV and the Xe-lamp′s discharging pulse width is 0.51 ms. Experiments are designed to research the effect of lateral amplified spontaneous emission (ASE) and luminous fefficiency of Xe-lamp on the small signal gain coefficient.