Abstract: In order to study the distribution of spontaneous noise in a Yb3+ -doped fiber amplifier (YDFA),the power gain characteristics of amplified spontaneous emission under conditions of different pump schemes and power,end surface reflectivity and spectral filter were studied by solving the discrete rate equation.The results show that the power of amplified spontaneous emission is different under different boundary conditions,and that the power amplification of the amplified spontaneous emission can be suppressed effectively by reducing the fiber surface reflectivity and using a properly selected spectral filter.