Based on the energy-level structure of quasi-three-level Yb ion, an amplified spontaneous emission（ASE） model in pumping process is set up. The stored energy density distribution and the energy that can be extracted in Yb:YAG crystal are calculated. The effect of ASE on energy-storage is investigated specifically, which includes the aperture, doping concentration and thickness, and working temperature of the gain medium. This paper gives an optimizing design of a pulsed energy-storage rep-rated Yb:YAG disk laser amplifier, which will be helpful in designing high-energy diode-pumped solid-state lasers based on Yb-doped materials.