In this paper, the 3 – dimensional ASE calculation for electron beam pumping XeCl excimer laser is presented by M – C method. In this model wall – reflected ASE is included. This calculation also includes nonsaturable absorption and mirror that reflect ASE flux back into the active gain medium. Results are presented that show optimum scaling of injected flux. It can provide theoretical basis and experimental references for experiments on excimer lasers, and be extrapolated to any other type of laser.