A model to predict the xenon flashlamp output spectrum, LC net circuit and inversion quantity during the pump has been established. The model has been examined by calculated and experimental results. The input power per unit wall area is more useful to describe the output spectra of flashlamp than current density which is used by previous studies. The spectral efficiency of flashlamp will fall as the input power is increased. It is concluded that the instantaneous output spectra and radiant efficiencies of flashllamp with different bore diameters are nearly identical at quasi stationary conditions compared with that at the same input power per unit wall area of the bore. It is also shown that (1) spectral efficiency and radiant efficiency of flashlamp will decline at the same current density as the bore diameter of flashlamp is increased, (2) peak current density of step line pump equals to 90 percent of one net circuit at the same pump width and so with higher efficiency, (3) inversion efficiency will adds 5 percent when pump width reduces 10 percent, which is good agreement with experiment.