We have developed high power atomic iodine laser amplif er with an aperture of 320 mm and length of 1.4m. Its conversion efficiency reaches to 0.8%. The cross section of the amplif er is better than 4%. The diffraction index change during the pumpimg period is less than 2x 10-7. Experimental results show that the empirical formula D P=20000(Pa.cm), in which P is the pressure of active material C3F7I inside the laser tube and D the diameter of an amplifer, is suitable for choosing parameters of amplifera.