The Φ350 mm amplifier of No.9 laser of SHENFGUANG II demands the high gain coefficient, the good beam quality, the safety and stable running and the low cost. The gain coefficient has been raised by the improvement of pump lamps form and energy feed system, the design of Nd:glass, the suppression of the parasitic oscillation, the use of corner reflector, protective glass with sol-gel anti reflection film and high efficiency xenon lamps. The beam quality has been improved by the non-stress install of Nd:glass disks. The stable running has been enhanced by the improvement of cavity merit rating. The amplifier has been manufactured and installed. The characteristics of No.9 system have reached the design requirement.