According to the problem of gain medium cross section high illumination and non-uniformity, which is caused by the laser rod amplifier non-imaging pump style. Orthogonal test design method is used to research the effect of absorption coefficient, gain medium radius, number of xenon lamps, center distance of xenon lamps and gain medium on gain uniformity. The rod amplifier which is made up by the above four elements, its gain medium cross section illumination distribution is simulated by ASAP. The results show that center distance of xenon lamps and gain medium, number of xenon lamps have very little influence on the gain uniformity. When absorption coefficient equal to 5.1 cm-1, gain uniformity will reach the optimum. Under the circumstance of other three elements are equal, the bigger is the gain radius, the smaller slope reflection curve, and the better gain uniformity.