For the laser system performance, the resonator optimum transmission is of great important. The optimum transmission of low gain, broadband-pulsed excimer laser is closely relative to the gain duration, which is caused by its pump pulse. The transition behavior of excimer laser is simulated based on the rate equations of four-level laser energy systems. In terms of reasonable approximation, an analytical description for the optimum transmission of the low gain,pulsed excimer laser is developed. The uniform optimum transmission condition in the different gain duration is given, and the optimum transmission as a function of net gain is analyzed. The optimum transmission of low gain, broadband-pulsed excimer laser is fundamentally different from the continuous wave (CW) lasers, and it depends on the relation of gain duration and photon lifetime in the resonator, these above mentioned analytical results are approved by the corresponding experimental results.