This paper reports the experimental measurements of gain characteristics of a XeCl laser pumped by Blumlein fast discharge device, using the method of varing output coupling, according to the relation between output power of the laser consisting of a stable cavity and homogeneously broadended gain medium, and other laser parameters, such as small-signal gain coefficient go, constant absorption loss coefficient saturation intensity Is and cavity length finally, the optimum transmittance of laser is determined. Authors find that the optimum transmittance is a function of pumping power.