To diagnose the beam quality of high power and large aperture laser, a system of attenuation and compression is designed and built to realize big minification and 2D compression and diagnosis by conventional beam quality dignosis systems such as Pyrocam Ⅲ. Average power is attenuated to 104 and compression ratio reaches 16 times of original beam aperture. The performance of the compression system irradiated by high power CO2 laser is simulated using Light Tools, and results indicate that this transformation method is simple and exact, which can realize the design targets efficiently.