Based upon the instantaneous output spectra of a xenon flashlamp, we have constructed a unique dynamic modeling program to predict the gain performance of multi segment Nd:glass laser amplifiers. The model, which includes all the dynamic modulations from flashlamp discharging to laser output, may be used in the laser amplifier system optimization. The gain uniformity in the horizontal direction is investigated with the modeling program. The amplified spontaneous emission inside a laser slab first increases the overall decay rate and thereby decreases the stored energy at the slab edges. The stored energy density at the slab center is higher than that at the slab edges when the explosion fraction of the flashlamp becomes large. The influences of the disk thickness and Nd2O3 doping level on gain performance are also studied with the modeling program. The stored fluence is determined by the doping-thickness products (Nd ion concentration ×slab thickness)in the multi-segment amplifiers slab, and the details are also given.