Taking account of the transmission characteristics of the diaphragm and the saturation of the gain in the process of light amplifying, the evolution of the beam inside the Cassegrain multi-pass amplifier is numerically simulated by using the ray-tracing and split-step fast Fourier-Bessel transform (SSFFT) methods, the annular beam is coming from Q-switched Nd∶glass laser with ring cavity. Preliminary experimental verification has been carried out. Analysis indicates that the staircase-like features of the input and output mirror reflections are responsible for the poor quality of the output annular beam, and special attention has been paid on optimizing amplifier configuration. As a result, a new type of amplifier configuration is proposed, whose input mirror has an anti-super Gaussian reflection distribution and output mirror has a super-Gaussian reflection distribution. The simulated results indicate that the use of the mirror couple can improve the quality of the output annular beam considerably.