Abstract: The relationship between the net gain of two kinds of glass with high dopant concentrations and pumping power,thread lengths is measured at room temperature.The results show that,under 100mW pumping power,the net gain of Er-doped glass thread with 0.19g/cm3 concentration is 1.96dB/cm,its threshold pumping power is 36mW and optimum length is 4.5cm.The net gain of Yb:Er-codoped glass thread with 1.1g/cm3 Yb-concentration and 0.12g/cm3 Er-concentration is 3.07dB/cm,its threshold pumping power is 28mW and optimum length is 2.5cm.That is,10dB total net gain can be obtained within 10cm length.The net gain of Yb:Er-codoped glass threads don't take on the trend of saturation when pumping power becomes stronger.It means that more sensitive ytterbium improves the net gain characteristics of the Er-doped fiber amplifiers.