Amplified spontaneous emission (ASE) is a key issue which affects the gain of large-aperture Ndglass slab amplifiers. Gain measurement experiments under different pump conditions are carried out to study the effect of ASE on gain coefficients and gain distributions. The gain coefficient derating ratio caused by ASE is calculated based on the experimental data. The results show that the gain turning-down and the decrease of average gain becomes notable as the pump energy increases. The study provides experimental support for further optimization of home-made large-aperture slab amplifiers.