Abstract: In this paper,the static transmission of phosphate laser disk glass used in high power laser amplifier is measured. In normal research condition,the average static transmission at 1.054μm incident beam with four pieces of phosphate laser glass(N31) disposed by Brewster angle is 94.5%,the loss coefficient is 0.294% cm-1,the average non-activity absorption coefficient is 0.146% cm-1,this N31 glass is made in China. Based on them,the small signal gain coefficient value is amended and reached up to 5.25% cm-1.