Reflective beam expanders are widely used for high-power laser launchers. However, compared with refractive expander, it is harder to realize zoom effects because of off-axis. This paper focuses on a new program, using one piece of main mirror and two different secondary mirrors to change the magnification of expanders, and a system is designed to expand a CO2 laser beam from 54 mm diameter to 81 mm (1.5×) and 162 mm (3.0×) in the state of off axis. The simulation results indicate that this kind of design is of great practicality with almost the same modulation transfer function (MTF) curves as a maladjusted telescope, having the same order of magnification with commonly used expanders. After compensating the defocus amount, the system′s imaging achieves the diffractive limit.