Abstract: The interaction of amplified spontaneous emission (ASE) flux and coherent laser flux as well as the effect of ASE in cuboid DF amplifiers are studied, by using a finite difference method and an iterative algorithm. The influence of input laser intensity and saturation light intensity on output power and energy extraction efficiency is discussed. The simulation of the amplifier whose gain zone is cuboid and gain coefficient is distributed as parabola indicate that the ASE intensity will decrease with the increase of input laser and the presence of input laser will make the maximum of ASE flux in the amplifier shift from the center to the entrance of the amplifier. The output power and the energy extraction efficiency will increase with the increase of input laser power when the saturation l