The energy storage in the Cr4+, Yb:YAG crystal amplifier is stimulated under the conditions of atom fraction thickness product 15%·mm and pumping density 20 kW/cm2 for different apertures and doped Cr4+ and Yb3+ densities, using the pumping dynamic model for Cr4+, Yb:YAG crystal amplifier. The results indicate that the density of energy storage decreases with the increasing of Yb3+ and amplifier aperture in absence of Cr4+; but the doped Cr4+ in Yb:YAG crystal affects on the energy storage in the amplifier. In some cases the maximum energy storage in amplifier increases first, then decreases with the increasing of Cr4+ density. Namely, for maximum energy storage there is an optimized Cr4+ density, which is determined by the Yb3+ density and aperture of amplifier.