The expander lens can match the beam dimension between two cascade amplifiers, so the laser medium can be used effectively and the laser output energy can be improved greatly. Study on the functions of the expander lens in high energy Nd:Glass laser is presented. It is shown that the output energy increases by 30% with the expander lens. The simulation through using the optical software CodeV indicates that adjusting the expander lens can effectively compensate the laser thermal lens, make the focusing beam into the parallel beam, and avoid damaging the optical elements. In addition, adjusting the expander lens may compress the laser beam divergence and focus the laser spot, which is beneficial to form high power density and meet the requirements of laser shock processing.