In this paper the transport equations of iaser and ASE normalized fluxes are set up taking into consideration the influence of ASE on threshold condition and the output characteristics of XeCl laser oscillator. The analytic expressions of laser and ASE fluxes distribution and output power are obtained. The results show that: (1) The threshold gain increases in consideration of ASE. (2) The ASE output decreases with cavity Q increases. (3) ASE flux can be syppressed effectively by choosing reflectivity of cavity mirror adequately. This has been proved by the measurement on the space coherence of XeOl excimer laser.