The influences of some factors of rod amplifier on its pumping efficiency are studied by numerical methods with certain radii of the rod and the lamps. These factors include the refectivity of the inside of its elliptic cavities, the cavities′ major and minor axises, the distance of the rod away from the central focus to the lamps, the number of the lamps and the structure of the glass tube for cooling the rod. The results are very useful for the design of rod amplifier.