High power pulse xenon lamps are usually used as the pumping source for slab amplifiers in high power laser facilities. The spectrum of xenon lamps is strongly linked to the gain performance of slab amplifiers. A spectrum testing system in a dark room with the time resolution of 2 μs is built to test the xenon lamp spectra at different discharging time. The results show that the xenon lamp gives quite different spectra for rising and falling edges of the current waveform with the same current density. The rising edge spectrum has many obvious discrete spectra. If no pre-ionization is applied, the radiation spectrum energy at the rising edge is 48% lower than that at the falling edge when the current density is 0.866 kA·cm-2. If pre-ionization is applied, the difference reduces to 24%. The pre-ionization can effectively increase the radiated energy of the rising edge, and the pre-ionization is an effective way to increase the radiation efficiency of the xenon lamp. The original xenon lamp radiation spectrum model is modified to agree with the experimental spectral data well.