As one of the most important element in the inertial confinement fusion (ICF) laser device, slab amplifier provides over 99% energy. The integration-test-bed (ITB) is established for the research of ICF in China with the single- beam output energy up to 19.6 kJ (pulse width 5 ns, center wavelength 1053 nm). This paper studies characteristics of N31 neodymium glass in 400 mm aperture, single-segment amplifying system of ITB. The N31 glasses with size of 810 mm×460 mm×40 mm and Nd³⁺ ion concentration of 3.5×10²⁰ cm^-3 are used. Results show that combined with the optimized parameters of discharge circuit for slab amplifier and structure of pump cavity, the small signal gain coefficient reaches 5.28% cm^-1, the gain ratio is 15:1 , and the gain uniformity is 1.063∶1 (maximum/ average) within 360 mm beam diameter.